On a spit of land on the northern side of Portsmouth Harbour are the remains of this castle, plus a Norman church. Built in the late 12th century by King Henry II, the castle is set inside the walls of a 3rd-century Roman fort built as a defense against Saxon pirates, when this was the northwestern frontier of the declining Roman Empire. This is the most complete set of Roman walls remaining in northern Europe. However, the castle itself is long gone. But the central tower remains, and from it you can take in a panoramic view of the harbor and the rocky coast.