Britain's Maunsell Sea Forts [wiki] were built during WWII as part of the coastal defense system. They were decommissioned in the 1950's, but many of them remain in use for non-military purposes (this is arguably the most famous). Some great photos here. [previously on metafilter]
In the South of France you'll find the fortified city of Carcassonne, a UNESCO World Heritage site, and a classic example of the medieval fortified city. Built upon the ruins of forts that predate Christianity, Carcassonne is one of the most photogenic places I've ever seen, never more so than on Bastille Day, when the city sets the night sky ablaze. A full gallery of Carcassonne fireworks can be found here.
The Maunsell Sea Forts: During the Second World War, three anti-aircraft forts were built in 1941-42 to protect the Thames Estuary, designed by Mr. G. A. Maunsell.