Cemlyn Bay Roman Fortlet
Examine the cropmark, geophysical evidence and strategic coastal position of the first early Roman military site identified on Ynys Môn.
Section I
Historical Context
Understanding why Mona mattered requires grasping its unique position at the intersection of religion, trade, and resistance in pre-Roman Britain.
01
Religious Centre
Mona served as the primary centre of druidic learning and practice in Britain, comparable to a great university. It took up to ten years of study to reach the rank of Chief Druid. The island's deep pools and ancient oak forests made it an ideal environment for druidic rites.
02
Trade Route
Mona occupied a critical position on the trade route from Ireland, particularly for gold mined in what is now County Wicklow. This precious metal was in constant demand by Rome, and controlling Mona would secure this valuable western corridor.
03
Natural Resources
The island contained one of Europe's largest copper mines at Amlwch (Parys Mountain) on the northeastern coast, with workings dating back to the Neolithic period. Roman-stamped copper ingots found on the island confirm systematic exploitation after the conquest.
04
Political Influence
The druids wielded considerable political influence among the Celtic tribes, serving as advisors to tribal leaders and providing ideological cohesion for anti-Roman sentiment. Destroying Mona was as much a political act as a military one.
The First Attempt: Suetonius Paulinus (60–61 CE)
The first Roman attempt to conquer Mona occurred in 60–61 CE under Governor Gaius Suetonius Paulinus. He constructed a flotilla of flat-bottomed boats to cope with the shallows of the Menai Strait, sending infantry across by boat and cavalry by fording or swimming alongside their horses.
On the beach stood the adverse array, a serried mass of arms and men, with women flitting between the ranks. In the style of Furies, in robes of deathly black and with dishevelled hair, they brandished their torches; while a circle of Druids, lifting their hands to heaven and showering imprecations, struck the troops with such an awe at the extraordinary spectacle, that, as though their limbs were paralysed, they exposed their bodies to wounds without an attempt at movement.
Tacitus, Annals, XIV.30Despite initial success, Suetonius was forced to abandon the campaign when news reached him of Boudicca's rebellion in the southeast. This hasty withdrawal allowed the druids and local Celtic population to maintain their independence and religious practices on Mona for another seventeen years.
The Intervening Years (61–77 CE)
Between Suetonius's withdrawal and Agricola's campaign, five Roman governors administered Britain, each focusing on different priorities. Petronius Turpilianus (61–63 CE) adopted a conciliatory approach after Boudicca's rebellion; Trebellius Maximus (63–69 CE) governed during a period of civil war in Rome; Vettius Bolanus (69–71 CE) maintained a defensive posture; Quintus Petillius Cerialis (71–74 CE) campaigned against the Brigantes in northern England; and Sextus Julius Frontinus (74–77 CE) subdued the Silures in South Wales. Throughout this period, Mona remained a symbol of native resistance and a refuge for anti-Roman elements.
Section II
Agricola: Background, Strategy & Planning
Gnaeus Julius Agricola was uniquely qualified for this campaign — a man who had witnessed the first attempt and spent years preparing for the second.
The Man Behind the Campaign
Born in Gallia Narbonensis (modern southern France) in 40 CE to a senatorial family, Agricola had previously served in Britain as a military tribune under Suetonius Paulinus and had witnessed the first attempt to conquer Mona. He later served as quaestor in Asia, tribune of the plebs, and praetor before returning to Britain as commander of the Twentieth Legion (Legio XX Valeria Victrix). He was appointed governor of Britannia in 77 CE by Emperor Vespasian.
Tacitus, who was Agricola's son-in-law and our primary source, presents him as a courageous and aggressive leader. Before even turning to Mona, Agricola first dealt with the Ordovices on the mainland, who had destroyed nearly an entire squadron of Roman cavalry. He "collected a force of veterans and a small body of auxiliaries; then as the Ordovices would not venture to descend into the plain, he put himself in front of the ranks to inspire all with the same courage against a common danger, and led his troops up a hill. The tribe was all but exterminated."
The hillfort of Dinas Dinorwig, located some 3.5 km from the Menai Strait in Arfon, is a strong candidate for the site of this engagement. Its name is traditionally interpreted as "Fort of the Ordovices," and it is the largest and best-defended hillfort in the region, commanding a wide view of the surrounding landscape. Agricola's victory here left him within striking distance of the strait and the island which he and his veterans must have remembered.
Strategic Objective 1
Eliminate druidic influence — complete the Roman policy of eradicating druidism, which was seen as the ideological engine of native resistance.
Strategic Objective 2
Secure the western flank before advancing northward against the Caledonian tribes in modern Scotland.
Strategic Objective 3
Control resources — secure access to the island's mineral wealth, particularly copper, and dominate the Irish Sea trade routes.
Strategic Objective 4
Demonstrate Roman power — achieve what Suetonius had failed to accomplish, thereby proving the inevitability of Roman rule to other resistant tribes.
Logistical Preparations
Unlike Suetonius's hastily assembled flotilla, Agricola's campaign was characterised by meticulous preparation. He gathered intelligence on the tidal patterns of the Menai Strait, established a staging base at Segontium (modern Caernarfon), and coordinated with the Classis Britannica (Roman British fleet) for naval support. Most critically, he trained auxiliary troops — recruited in Britain — who were experienced swimmers, capable of crossing the strait without boats. This was to be the key to tactical surprise.
Section III
The Crossing of the Menai Strait
The most debated question in the archaeology of this campaign: where exactly did the Romans cross, and how?
The Menai Strait is a narrow strip of tidal water approximately 25 km long separating Ynys Môn from the Welsh mainland. At full strength, tidal currents can reach up to 7.5 knots, with considerable turbulence in the narrower and rockier sections. The crossing would be short and relatively easy at slack water, but timing was everything.
The skill and resolution of the general accomplished the passage. With some picked men of the auxiliaries, disencumbered of all baggage, who knew the shallows and had that national experience in swimming which enables the Britons to take care not only of themselves but of their arms and horses, he delivered so unexpected an attack that the astonished enemy who were looking for a fleet, a naval armament, and an assault by sea, thought that to such assailants nothing could be formidable or invincible.
Tacitus, The Life of Gnaeus Julius Agricola, Chapter 18The defenders of Mona expected to see a fleet in plenty of time to gather for their defence. Agricola's use of swimming auxiliaries, without legionaries and without boats, was a deliberate act of tactical deception that demoralized the island's leadership to the point of submission. The exact crossing point remains unknown and modern suggestions are speculative, but two principal candidates have been identified by historians and archaeologists.
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Route A
Lavan Sands / Bangor Flats
Location
NE Strait, near Bangor
Sea crossing at low tide
~240 metres
Sand flat approach
4.5 km (Lavan) / 1.5 km (Bangor Flats)
Current strength
Strong (up to 7.5 knots)
Landing terrain
Steep slopes; Gallows Point offers flat ground
Access from mainland
Excellent — via Sychnant Pass
Historical use
Medieval ferry crossing until 1826
Route B
Llanidan / Porthamel
Location
SW Strait, Ynys Môn shore
Sea crossing at low tide
~350 metres (min)
Sea crossing at high tide
~850 metres
Current strength
Less violent; less turbulence
Landing terrain
Gentler slopes; easier landing
Access from mainland
Requires crossing high ground or valley
Field name evidence
Maes Hir Gad & Cae Oer Waedd (1867)
The Agricola Connection to Llanidan
An army coming from Dinas Dinorwig — the probable site of Agricola's defeat of the Ordovices — might have moved most directly, some 3 km, to the south-western reaches of the Menai Strait opposite Llanidan. This geographical logic, combined with the field name evidence recorded in 1867, makes the Llanidan route a compelling candidate specifically for Agricola's crossing, as opposed to Suetonius's earlier attempt which may have used the Lavan Sands route.
Henry Rowlands, writing in the 18th century, identified a ford at Porthamel in Llanidan parish, across the Menai Strait from the present site of Felinheli (Y Felinheli), where he suggested the Roman cavalry had swum across. He noted a place called "Pant yr yscraphie" nearby, arguing the Romans' flat-bottomed boats (scaphae) had landed there, and that a nearby tumulus marked the site of a great druidic sacrifice.
Section IV
Archaeological Evidence
Decades of excavation and survey have begun to fill in the gaps left by Tacitus, revealing the physical footprint of Roman Mona.
Discovered in 1942–43 during wartime construction at RAF Valley, the Llyn Cerrig Bach hoard is one of the most significant Iron Age finds in Britain. Dredged from a small peat-filled lake on the western side of Ynys Môn, the assemblage comprises approximately 180 items of metalwork, including bronze swords, spears, shield bosses, bronze horns, cauldrons, slave-gang chains, iron currency bars, and chariot fittings.
The chronological range of the finds, spanning from the 4th century BC to the 2nd century AD, strongly suggests a site of continuous ritual deposition over at least 500 years. The overwhelmingly military character of the finds — swords, spears, chariot fittings — is consistent with votive offerings made by a warrior aristocracy. The presence of slave chains is particularly striking: five hinged neck rings joined by figure-of-eight links, capable of holding five captives some 60 cm apart.
No sacred groves have been confirmed on Ynys Môn by archaeology, but Llyn Cerrig Bach provides compelling physical evidence for the kind of druidic ritual practice that so alarmed the Romans. The abrupt end of deposition at the time of the Roman conquest is itself a form of archaeological testimony to the campaign's impact.
The Cemlyn Cropmark, located on a low hill overlooking Cemlyn Bay on the north coast of Ynys Môn, is the first Roman military site on the island dated to around the time of Agricola's invasion in 77 CE. It was first spotted by aerial photographer Mary Aris in 1990 during a long dry spell, when the buried remains showed as a circular mark in crops.
In 2015, Gwynedd Archaeological Trust secured funding from Cadw for a geophysical survey, which revealed the "unmistakably clear" outline of a Roman fortlet — a smaller version of a Roman fort. Partial excavation of the site found two coins, one from Nerva's reign and one from Hadrian's, supporting the dating of the fortlet to post-invasion consolidation rather than the invasion itself.
The fortlet overlooks Cemlyn Bay, a good landing place on the north coast of the island, and would most likely have served as a guide for landing sea-craft as well as a policing station for those wanting access to Ynys Môn. David Hopewell of Gwynedd Archaeological Trust noted that fortlets are usually linked by roads 15 to 20 miles apart, and it is hoped the discovery will lead to further finds.
The settlement at Tai Cochion sits close to the Ynys Môn shore of the Menai Strait, directly opposite Segontium (Caernarfon). Metal detectorists had been reporting finds of large amounts of Roman metalwork from the surrounding fields for years, and regular reports of Roman finds in the area were also made to the Cambrian Archaeological Association during the 1860s.
Geophysical surveys by Gwynedd Archaeological Trust, followed by community excavations in 2010 and 2011, revealed an extensive settlement comprising a 0.6 km long road with several side branches running from Tai Cochion house to the shore of the Menai Strait. The roads were flanked by a series of small enclosures, each containing rectangular buildings of approximately 16 m × 8 m — a Roman layout quite distinct from the traditional circular huts found elsewhere on the island.
Close to 1,000 sherds of pottery were recovered along with several coins from stratified contexts. Preliminary analysis suggests occupation from the early to mid 2nd century to the mid 3rd century CE. The settlement appears to have developed around the crossing point from Segontium, functioning as a civilian trading point — the first such small town identified in north and mid Wales.
Segontium (modern Caernarfon) was founded by Agricola in AD 77 or 78 after he had conquered the Ordovices in North Wales. It was the main Roman fort in the north of Roman Wales, designed to hold approximately 1,000 auxiliary infantry. It was connected by a Roman road to the legionary base at Chester (Deva Victrix), and its elevated position gave a commanding view of the Menai Strait.
The original timber defences were rebuilt in stone in the first half of the 2nd century. A large courtyard house with its own small bathhouse was built within the fort, possibly the residence of an official in charge of regional mineral extraction. Archaeological research shows that by the year 120, there had been a reduction in military numbers, suggesting the island was considered pacified. By the 3rd century, Segontium was garrisoned by 500 men from the Cohors I Sunicorum, originally levied in Gallia Belgica.
Coins found at Segontium show the fort was still occupied until at least 394 CE. The remains of a civilian settlement, a Roman temple of Mithras (the Caernarfon Mithraeum), and a cemetery have also been identified around the fort, testifying to the long-term Roman presence in the area.
Continue with The Roman Invasion of Mona, or browse the Roman Britain research archive for related Segontium and Mona material.
Copper had been mined on Parys Mountain (near Amlwch) for centuries before the Roman arrival, with workings dating back to the Neolithic and Bronze Ages. The site represents one of the largest prehistoric copper mines in Europe. Roman involvement in the industry is confirmed by the discovery of some 18 round ingots of copper on the island, eight of which bear Roman stamps, confirming that production was organised and regulated by Roman authorities post-conquest.
The two smelting sites are unknown, but the ingots themselves demonstrate that copper production — possibly increased for export from the island — continued under Roman rule. A high-status building within Segontium fort, interpreted as the residence of an important official, may have been the residence of the official in charge of this regional mineral extraction programme.
In the last decades of Roman rule, several military installations were built on the northern and western coasts of Ynys Môn to defend against Irish sea raiders. Caer Gybi at Holyhead is the most substantial, a shore-fort whose walls remain visible today. It was created in the mid-4th century as a base to secure the island against seaborne threats from Ireland.
Pen Bryn-yr-Eglwys, a rocky hilltop on the north-western corner of Ynys Môn, bears a square stone platform measuring 9 m by 7 m. Its positioning, small size, and three sherds of probably-Roman pottery strongly suggest it was a Roman watchtower. A similar site, also interpreted as a watchtower, has been excavated inside the pre-Roman fort of Caer y Twr on Holyhead Mountain. There is a possible second fortlet on Bwrdd Arthur, towards the eastern end of the north coast.
Key Sites at a Glance
| Site | Type | Location | Date | Significance |
|---|---|---|---|---|
| Llyn Cerrig Bach | Ritual | Near RAF Valley, W. Ynys Môn | 4th c. BC – 2nd c. AD | Massive votive deposit of weapons, slave chains & chariot fittings; confirms pre-Roman druidic activity |
| Cemlyn Fortlet | Military | Cemlyn Bay, N. Ynys Môn | c. 77 CE+ | First Roman military site on the island; discovered by aerial photography 1990, confirmed by geophysical survey 2015 |
| Tai Cochion | Civilian | Llanidan, Menai Strait shore | c. 100–300 CE | First civilian Roman small town in N. Wales; marks the Menai crossing point opposite Segontium |
| Segontium | Military | Caernarfon, mainland | 77/78 CE – c. 394 CE | Founded by Agricola; administrative hub for Roman Ynys Môn; held ~1,000 auxiliary infantry |
| Parys Mountain | Resource | Amlwch, NE Ynys Môn | Neolithic – Roman | One of Europe's largest prehistoric copper mines; 8 Roman-stamped ingots confirm post-conquest exploitation |
| Caer Gybi | Military | Holyhead | Mid-4th c. CE | Shore-fort defending against Irish raiders; walls still visible; later became site of St Cybi's Church |
| Pen Bryn-yr-Eglwys | Military | NW Ynys Môn | Roman period | Probable Roman watchtower; 9 m × 7 m stone platform with Roman pottery sherds |
| Dinas Dinorwig | Military | Arfon, 3.5 km from Menai Strait | Iron Age | Largest hillfort in Arfon; probable site of Agricola's defeat of the Ordovices before the Mona crossing |
Section V
Welsh Place Names & Linguistic Evidence
The Welsh language has preserved the memory of these events for nearly two millennia. These names are not mere curiosities — they are primary sources in their own right.
Maes Hir Gad
"Area of the Long Battle"
Llanidan Parish, SW Ynys Môn
Recorded in 1867 in the journal Archaeologia Cambrensis, this field name in Llanidan parish is one of the most compelling pieces of linguistic evidence for the location of the Roman landing. The name implies a prolonged engagement, consistent with a contested beach landing rather than a swift rout.
Cae Oer Waedd
"Field of Cold / Bitter Lamentation"
Llanidan Parish, SW Ynys Môn
Recorded alongside Maes Hir Gad in 1867, this name evokes the human cost of the confrontation from the native perspective. The "cold lamentation" may refer to mourning after battle, or to the ritual grief of a community that had lost its sacred centre. It is a uniquely human monument to the event.
Porth y Wygyr
"Port of the Warriors"
Cemaes Bay, N. Ynys Môn
Located on the northern coast of Ynys Môn, this place name may reference the Roman military presence or the native resistance during the campaign. Its position near Cemlyn Bay — where the Roman fortlet was later discovered — lends it additional significance.
Caer Gybi
"Fort of Cybi"
Holyhead (Caergybi)
While named after a later Christian saint, this site at Holyhead contains Roman fortifications dating to the mid-4th century CE. The Roman walls survive and are visible today. The continuity of use from Roman fort to Christian church reflects the broader pattern of cultural layering across the island.
Segontium / Caer Segeint
"Strong Place" (Brythonic *seg-ontio)
Caernarfon, mainland
The Roman fort's name survives in the Old Welsh form Cair Segeint, listed among the 28 cities of Britain in the Historia Brittonum. The modern name Caernarfon itself derives from "Caer yn Arfon" — "fort in the land facing Môn (Ynys Môn)" — a direct linguistic reference to the fort's purpose of overseeing the island.
Porthamel
Ford / Landing Place
Llanidan Parish, SW Ynys Môn
Identified by Henry Rowlands in the 1700s as the probable ford where Roman cavalry swam across the Menai Strait. Rowlands also noted a place called "Pant yr yscraphie" nearby — arguing the Romans' flat-bottomed boats (scaphae) had landed there, with a nearby tumulus marking the site of a great druidic sacrifice.
The Significance of Field Names as Evidence
Field names recorded in the 19th century — particularly those in the parish of Llanidan — occupy a unique evidential position. They are not literary sources, not archaeological finds, and not topographical features: they are the accumulated memory of a farming community, passed down through generations in the Welsh language. The fact that names such as "Maes Hir Gad" and "Cae Oer Waedd" were still in use in 1867 — nearly 1,800 years after the events they commemorate — speaks to the depth of cultural memory embedded in the Welsh landscape.
These names do not prove the location of the Roman crossing, but they corroborate it. Taken together with the topographical analysis of the Menai Strait, the archaeological evidence from Tai Cochion, and the geographical logic of Agricola's approach from Dinas Dinorwig, they form a coherent body of evidence pointing to the Llanidan area as a significant locus of the Roman campaign.
Section VI
Deep Analysis
Three lenses through which to understand the campaign: what happened, why it mattered, and what it meant to the people who lived it.
I
What Happened
The Strategic Execution
Agricola's campaign was a masterclass in Roman military planning. By first destroying the Ordovices on the mainland, he secured his rear and demoralised potential allies on the island. His use of swimming auxiliaries — rather than a fleet — denied the defenders the warning they needed. The crossing was swift, the resistance was overcome, and the island surrendered without the prolonged siege that Suetonius had faced.
II
Why It Mattered
Strategic & Cultural Transformation
The conquest of Mona secured the western flank of Roman Britain, enabling Agricola's subsequent campaigns into Caledonia. It integrated the island's mineral wealth — particularly copper — into the imperial economy. Most profoundly, it destroyed the intellectual and spiritual heart of Celtic Britain, accelerating Romanisation and dealing a fatal blow to organised druidism as a political force.
III
The Human Perspective
Memory in the Landscape
For the Celtic inhabitants of Mona, this was not a strategic manoeuvre but a catastrophe. The field names that survive — "Field of the Long Battle," "Field of Cold Lamentation" — are the voices of those who had no Tacitus to record their story. They remind us that the landscape of Ynys Môn is not merely a geographical feature, but a palimpsest of human experience, grief, and memory.
The Reliability of Tacitus
It is worth noting that Tacitus's account of Agricola's campaign must be read with care. Tacitus was Agricola's son-in-law, and his biography of the governor is an explicitly laudatory work. His account of the Mona campaign is notably less detailed than his vivid description of Suetonius's earlier attempt, and his reliability should be carefully considered. Nevertheless, his account remains the only written record of the second invasion, and where it can be tested against archaeological and topographical evidence, it has generally proved consistent.
What the Archaeology Tells Us That Tacitus Does Not
The archaeological record reveals aspects of the Roman occupation that Tacitus entirely ignores. The Cemlyn Fortlet shows that the Romans immediately established military control of the island's coastline. The Tai Cochion settlement demonstrates that a civilian economy quickly developed around the Menai crossing point. The Roman-stamped copper ingots from Parys Mountain confirm that the island's mineral resources were systematically exploited. And the absence of major civic centres or villas — despite three centuries of Roman rule — tells us that Ynys Môn was never fully Romanised in the way that southern Britain was. It remained, in some sense, a frontier.
The Llyn Cerrig Bach hoard, meanwhile, tells us what was lost. The 500-year sequence of ritual deposition — weapons, slave chains, chariot fittings — came to an abrupt end with the Roman conquest. Whatever ceremonies had been performed at that lake, whatever prayers had been offered, whatever sacrifices had been made: they stopped. The silence of the archaeological record after the conquest is itself a form of testimony.
Section VII
Outcomes & Long-Term Significance
77–78 CE
Conquest of Mona
Agricola defeats the Ordovices on the mainland, then crosses the Menai Strait with swimming auxiliaries. The island surrenders. Segontium founded as administrative hub. End of organised druidism as a political force in Britain.
c. 80 CE
Cemlyn Fortlet Established
A Roman fortlet is built overlooking Cemlyn Bay on the north coast, the first military installation on the island, serving as a coastal control point and landing guide for sea-craft.
c. 100–300 CE
Civilian Settlement at Tai Cochion
A Romanised civilian trading settlement develops on the Ynys Môn shore opposite Segontium, featuring Roman-style corridor housing and a metalled road — the first civilian small town in north Wales.
2nd century CE
Copper Production Expands
Roman-stamped copper ingots from Parys Mountain confirm systematic exploitation of the island's mineral wealth. A high-status official at Segontium likely oversees the extraction programme.
Mid-4th c. CE
Caer Gybi Shore-Fort Built
Growing insecurity from Irish sea raiders prompts the construction of the shore-fort at Holyhead. Watchtowers at Pen Bryn-yr-Eglwys and Caer y Twr are also established. Parciau hillfort shows a dense, defensive concentration of population.
c. 383 CE
Roman Military Control Ends
Magnus Maximus takes much of the garrison of Britannia to the Continent. Direct Roman military control of Ynys Môn effectively ceases. In 410, Honorius advises remaining Roman cities to look to their own defence.
1867
Field Names Recorded
B.A. Malaws records the field names "Maes Hir Gad" and "Cae Oer Waedd" in Llanidan parish in Archaeologia Cambrensis, suggesting they may indicate the site of a Roman attack — nearly 1,800 years after the events they commemorate.
1942–43
Llyn Cerrig Bach Discovered
Wartime peat-dredging at RAF Valley uncovers the Llyn Cerrig Bach hoard — approximately 180 items of Iron Age metalwork spanning 500 years of ritual deposition, providing the most vivid physical evidence of pre-Roman druidic practice on the island.
2015
Cemlyn Fortlet Confirmed
Gwynedd Archaeological Trust's geophysical survey confirms the Cemlyn Cropmark as a Roman fortlet — the first early Roman military site found on Ynys Môn, vindicating decades of searching for physical evidence of the Roman occupation.
Follow the wider chronology:
Continue from Agricola's campaign through rebellion, frontier war and the end of Roman rule in the
Roman Britain timeline.
Conclusion
Agricola's campaign against the druids of Mona represents a pivotal moment in the Roman conquest of Britain. Through careful planning, strategic insight, and the tactical innovation of a swimming assault, Agricola succeeded where Suetonius had failed, eliminating the last major centre of druidic practice and native resistance in Wales.
The evidence from historical accounts, archaeological findings, place names, and topographical analysis converges to support a reconstruction of the campaign that emphasises Agricola's methodical approach and exploitation of natural features — particularly the tidal patterns of the Menai Strait. The success of this campaign not only secured Roman control of western Britain but also accelerated the cultural and religious transformation of the native population.
The memory of this confrontation, preserved in Welsh place names and oral traditions, testifies to its significance in the historical consciousness of the region. Agricola's conquest of Mona stands as a testament to Roman military effectiveness and as a watershed moment in the cultural history of Britain, marking the effective end of organised druidism as a political and religious force in the British Isles. Yet the field names of Llanidan remind us that the people who lived through these events saw them very differently — not as a triumph of civilisation, but as a cold and bitter lamentation.
