Portchester Castle, Hampshire

English Heritage.
Portchester Castle is the most impressive and best-preserved of the Roman ‘Saxon Shore’ forts in Europe, and was built in the late 3rd century. It is the only Roman stronghold in northern Europe whose walls still mostly stand to their full six metre height, complete with most of the original 20 towers.
It later housed a Saxon settlement, and after the Norman invasion a Norman castle was built in one corner. The castle was used in the middle ages, and was used as a prison during the Napoleonic Wars.
The size of the circuit of Roman walls is impressive, and the medieval keep is worth seeing and has an interesting exhibition in the basement. Rooms in the keep can be reached by modern stairs. Note that in fair weather the more agile visitor can ascend a narrow spiral stone staircase to the roof, from which there should be great views of the harbour, etc. (I didn’t try it in the rain).
The castle is right on the waterfront, so don’t omit to walk out of the seaward gate and have a look.
Visiting: If you pass the obvious car park to the left, there is a smaller one on the right a few yards from the castle entrance. Admission to the Roman fort is free, but there is a charge for visiting the medieval keep. An audio guide is available. The keep has modern stairs to all floors except the roof (see above).