The Roman invasion of Britain in AD43 was led by a Senator Autus Plautius, with an army of some 50,000 men. Archaeological evidence suggests that Richborough was the bridge head for the invasion, and the pair of parallel ditches that can be seen running almost alongside the much later western stone walls of the site, were dug as part of the original fortifications in AD43. As the conquest of Britain rapidly advanced Richborough soon became an important naval supply base, as evidenced by the laying of new roads, and the erection of several timber buildings believed to be storehouses.
By AD85 the site was to undergo a substantial change. The central timber buildings were demolished, and a monumental four-way arch was constructed, the walkways beneath which formed the shape of a large cross that can still be clearly seen today. The main masonry structure would have stood in the corner spaces flanking the roadways and, judging by the extent of the foundations (some 88ft x 48ft or 26.5 x 14.5m), the arch would have been very substantial indeed, probably reaching a height of some 85ft (25m). Bronze and white marble fragments found at the site indicate that the archway would also have been quite ornate, and would have probably symbolised the entry into the province of Britain through what had now become its main sea port.
As Richborough's status grew, so did the settlement around it. However, towards the middle of the 3rd century, military considerations had again come to the fore. A large percentage of the central buildings were demolished and the central monument was ringed with a series of triple ditches, suggesting that this had become a useful lookout tower. These excavated triple ditches are still a striking feature of the site today. By the end of the 3rd century the earth fortifications were dismantled and the ditches backfilled to prepare for the construction of a much more substantial fortification enclosed by a stone perimeter. These massive walls with their corner towers, which were up to 11ft (3.5m) thick in places, surrounded by a double ditch, are the most impressive part of the ruins as seen today.
Little is known of the internal buildings, as these were most likely to have been constructed of timber. The changes are believed to have been completed by AD286. By the end of the 4th century Richborough Roman For had ceased to be garrisoned by regular Roman troops, but the site retained its status until approximately AD402. Coins found on the site confirm this, as does the discovery of a small Christian church. The hexagonal tiled basin that lies near the north perimeter wall would have been the baptismal font. The date of construction for the church is not known but it must have been in the late 4th century - early 5th century and was probably still in use after Richborough Roman For was abandoned.