- Saxon Shore Fort built in AD 280s – possibly by Marcus Aurelius Carausius, self-proclaimed emperor of Britannia and northern Gaul from AD 286-293.
- Guarded harbour, first against an invasion by Roman Emperor then against raids by Saxons, Jutes and Angles from the continent.
- After Roman withdrawal in the early fifth century, was home to a Saxon community.
- Large Norman keep and castle built in the fort after AD 1066.
- Became Royal residence of Richard II in AD 1390s.
- Occupied until 17th century.
- Prisoner-of-war camp until the end of the Napoleonic Wars.
- Most complete fort walls of any Roman fort in Europe. They enclose the entire fort area of 9 acres and stand at their original height, although upper level and parapets are medieval.
- 16 of 20 regularly positioned D-shaped towers still stand along the walls.
- Large Norman keep, moat and inner bailey occupies northwest quarter of fort.
- Later Royal castle built by Richard II attached to Norman keep.
- Medieval church of St. Mary and graveyard occupies southeast quarter of fort.