Portchester Castle was built within a Roman fort. Despite adaptation as a medieval castle, it is the best preserved Roman fort north of the Alps.Many castles were built on or within earlier sites of power.
Portchester Castle- The Norman keep, begun in 1067 and the keep added on in the 12th and 13th centuries to its present height. During the Napoleonic Wars the keep housed, in considerable discomfort, 7000 French prisoners. The Normans used an existing Roman fortress to build their keep in, this was built in 260's AD and remains the best preserved Roman fortress in England.