Portchester Castle in Fareham

You're standing at Portchester Castle, right at the head of Portsmouth Harbour. First, take a look at the long, low walls around you. They're not medieval at all, they're Roman. Probably between 285 and 290, the Romans built a fort here to fight off pirate raids, and called it Portus Adurni. It's said to be the best preserved Roman fort north of the Alps. The Roman fleet that defended Britain may well have operated from here.