Today I went on a one of the Museum of London’s periodic tours of the most  substantial surviving part of Cripplegate Roman Fort, preserved in the modern underground car park on London Wall.  The fort was originally built in around 120AD, and housed a garrison of perhaps as many as 1000 or more troops, including cavalry, on a 12-acre site to the north-west of the Roman city of Londinium.  Its  west and north walls  were subsequently incorporated  into the City Wall in around 200.  Part of the west wall, gate, and gate-house complete with guard-rooms and turrets, can still be seen in the modern car park, together with a fine reconstruction making sense of things.

Roman Fort Reconstruction (model)