What the Romans Built at Londinium: The Architecture of Roman London
Roman London was one of the most intensively built cities north of the Alps, housing the largest forum-basilica in the entire Roman world beyond the Mediterranean. Established within a decade of the Claudian invasion of 43 AD, Londinium grew from a riverside trading post into the undisputed capital of Roman Britain — a city of monumental public buildings, defensive walls, and infrastructure that would not be matched in Britain again for over a thousand years.
How Londinium Was Founded and Why It Grew So Quickly
The Romans did not build Londinium because a settlement already existed there. They built it because the Thames at that location was the lowest point at which the river could be bridged, making it the natural hub of the road network radiating across the new province. Within a generation of the invasion, Londinium had become the busiest port and most commercially active town in Roman Britain.
Its early growth was interrupted dramatically. Boudicca's rebellion of 60 or 61 AD burned the young city to the ground — archaeologists have recovered a distinct layer of red-scorched debris and destruction across much of the Roman city's footprint, a violent full stop on its first phase. Yet rebuilding followed swiftly, and it was in the post-Boudiccan decades that Londinium truly began to take monumental shape.
By the late 1st century, the city covered over 100 hectares. By the 2nd century, with a population estimated at between 30,000 and 60,000 people, it was the largest city in Roman Britain and the administrative capital of the province.
The Forum-Basilica: The Largest Building North of the Alps
The centrepiece of Roman London was its forum-basilica, and its scale is difficult to overstate. The second and final version of the complex, constructed in the early 2nd century, measured approximately 167 metres by 167 metres — making it the largest building of its type anywhere in the Roman world north of the Alps. Nothing comparable existed in Roman Britain, and few structures in the western provinces could rival it.
The basilica itself — the great hall used for administration, legal proceedings and commercial transactions — ran along the northern side of the forum courtyard. It was a multi-storey structure, likely over 25 metres in height at its apex, lined with colonnaded aisles and faced in imported stone and marble. The forum courtyard to the south was surrounded on three sides by ranges of shops and offices, creating a civic and commercial complex that announced, unmistakably, that this was an imperial city.
The remains lie largely beneath the streets of the modern City of London. Sections of the basilica wall were uncovered during construction work at Leadenhall Market and nearby sites, and a fragment of the basilica's massive masonry was preserved — somewhat incongruously — in the basement of a building on Gracechurch Street, where it can still be seen today.
The Governor's Palace, the Amphitheatre, and the Mithraeum
The forum-basilica was not Londinium's only monumental structure. The province's governor required a residence commensurate with his status as one of Rome's most senior appointed officials, and the building that has been identified as the governor's palace — the praetorium — was found during excavations at Canon Street and along the Thames waterfront in Southwark. It was a vast complex, possibly incorporating formal reception rooms, administrative offices and a garden courtyard.
Beneath the Guildhall Yard in the City of London lies the city's amphitheatre, discovered during works ahead of the Guildhall Art Gallery in the 1980s. Built initially in timber around 70 AD and later reconstructed in stone in the 2nd century, it could accommodate an estimated 6,000 to 7,000 spectators. The outline of the arena is now marked in the Guildhall Yard pavement, and sections of the eastern entrance survive beneath the gallery itself. Gladiatorial combat, animal hunts and public executions would all have taken place here — the amphitheatre was as much an instrument of Roman power as any administrative building.
The Temple of Mithras, discovered during construction work in 1954, caused a national sensation. The Walbrook Mithraeum — dating to the 3rd century AD — was a small but elaborately fitted cult building used by devotees of the mystery religion of Mithras, popular among soldiers and merchants. Sculptures of Mithras, Bacchus and Minerva were recovered from the site. The temple has since been relocated and is now open to visitors in its rebuilt form near Bloomberg's European headquarters.
The Roman Wall: Londinium's Defensive Circuit
Sometime around 190 to 225 AD, Londinium was enclosed within a substantial circuit of defensive walls — the Londinium town wall — running for approximately three miles around the landward sides of the city, with a later riverside wall added in the 4th century to complete the enclosure. Built from Kentish ragstone brought upriver by barge from quarries near Maidstone, the wall stood around six metres high and was reinforced by a series of bastions added in the later Roman period.
Much of this wall survives, in fragmentary but often substantial form, threading through the modern city — visible in stretches near Tower Hill, in the Museum of London's garden, and in fragments embedded in later medieval and post-medieval construction. The medieval and later City of London walls follow the same line almost exactly, meaning that Londinium's defensive boundary continued to shape the city for over a millennium after the legions departed.
Two Emperors in One City
That two reigning Roman emperors visited Londinium speaks to its strategic importance. Hadrian is believed to have visited in the 120s AD — his presence in Britain is attested by the construction of his famous wall in the north, and London would have been his principal base. Septimius Severus arrived around 208 AD to prosecute his northern campaigns, and died at Eboracum (York) in 211 AD. Londinium, as the provincial capital, would have been the administrative centre supporting both imperial visits — a fact that helps explain the scale of investment in its public infrastructure.
The Complexity of Researching Roman Sites
Understanding what Romans actually built at any given location in Britain requires navigating a wide range of sources: excavation archives, specialist site reports, aerial photography interpretations, finds databases, and a century of often contradictory antiquarian accounts. Cross-referencing these with modern development records and geophysical survey results is painstaking work, even for professional researchers. For those working on sites in or near historically Roman areas, Aubrey Research automates much of this process, pulling together the relevant historical and archaeological record into a structured report. You can see an example of what that looks like in the sample report.
Frequently Asked Questions
What was the largest Roman building in London? The forum-basilica of Londinium, constructed in the early 2nd century AD, was the largest building north of the Alps in the Roman world. The complex measured approximately 167 by 167 metres, and the basilica hall itself stood over 25 metres high.
Did Roman emperors visit London? Yes. Hadrian visited Londinium in the 120s AD, and Septimius Severus used it as a base for his northern campaigns around 208 AD. Two imperial visits to the same British city reflect Londinium's exceptional importance as the provincial capital.
Where can you see Roman remains in London today? Significant Roman remains are visible at Tower Hill (town wall), the Guildhall Yard (amphitheatre outline), and the rebuilt Walbrook Mithraeum near Bloomberg's London headquarters. Fragments of the forum-basilica survive in the basement of a building on Gracechurch Street.
When was the Roman town wall of London built? The main circuit of Londinium's defensive wall was constructed between approximately 190 and 225 AD, using Kentish ragstone quarried near Maidstone. A riverside wall was added in the 4th century to complete the enclosure. Much of this wall survives in fragmentary form throughout the modern City of London.