Portchester has grown up around the approaches to Portchester Castle. The Romans built a naval base here in the third century to guard against attack. Today, the walls are the most complete remaining example of their fortifications in northern Europe. There are spectacular views over Portsmouth Harbour and the remains of the Castle from the top of the Norman Keep. The castle is in Castle Street, Portchester. Inside is a fascinating interactive exhibition of England under Roman control. It is complemented by a series of colourful events and vivid re-enactments. There is also a shop.