This is a Norman keep built within the walls of a Roman Saxon shore fort. The Roman fort was built at the end of the 3rd century AD to defend the south coast of England against the Saxon invasions. The Normans reused the fort from the 11th century, Edward II fortifing it in the 14th century. Situated within Portsmouth harbour, the fort was used as the embarkation point for several medieval English invasions of France.
I didn't have a lot of time so didn't get to go inside, and it was freezing cold so the photos are not very good!
The Roman walls, right next to the sea.
The south gate
The church inside the walls
The Roman gateway from inside the walls
The gateway to the Norman castle inside the Roman walls
The Norman keep
Outside the walls