Fort Kijkduin

North Holland

Built under Napoleon's orders in 1811 to accommodate 1400 soldiers, this hulking hilltop fortress (originally called Fort Morland) now houses a military museum incorporating an armoury, and a fantastic subterranean aquarium with 14 tanks filled with every species of marine life from the Waddenzee and North Sea, one with a walk-through tunnel. Kids love it.

It's 4km east of Den Helder; take bus 31 (10 minutes, every 15 minutes).