It is part of a larger Roman town called Rutupiae or Portus Ritupis that developed around the fort and the associated port.

Because of its position near to a large natural harbour in the Wantsum Channel and to the mouth of the Stour, Rutupiae served as a main gateway to Roman Britain and the starting point for the road now known as Watling Street.

[2] The site expanded into a major civilian and commercial town, and the stone Saxon Shore fort was added around the year 277.

[4] An alternative attested name for the fort, Ritupiae, may represent a clearer British form, containing the word *ritus "ford" (Welsh rhyd), referring to a crossing point between the then island and the mainland.

[8] A major quadrifrons triumphal arch, one of the biggest in the Roman Empire, was erected in about AD 85 straddling Watling Street, the main road from Richborough to London.

Its position and size suggest it may have been built to celebrate the final conquest of Britain after Agricola's victory at the Battle of Mons Graupius.

This involved the demolition and reuse as spolia of the triumphal arch, and numismatic evidence suggests it occurred during the reign of Carausius.

Excavations in 2021 have revealed that the amphitheatre and the settlement are likely to have continued in use from the invasion to the end of Roman rule in the early 5th century.

A room, or cell, with walls almost 2 m high, used to hold wild animals, criminals or gladiators before entry in the arena was also found.

[16][17] Excavations from 2021 have focussed on the amphitheatre situated 300 m south west of the fort, where unique painted walls have been found.

From the top of the structure there are views all round the surrounding walls, and in good weather as far afield as Reculver to the north, the site of Regulbium, another Roman Saxon Shore fort.

In addition, the on-site museum has been refurbished to display some of the many finds from Richborough, a location which saw both the very beginning and the very end of Roman rule in Britain.

The American Russell Hoban repurposed Richborough Castle as "Roaming Rune" (alluding to its Roman origins) in his 1980, post-apocalyptic novel Riddley Walker.