Burg Rheinfels

Once the mightiest fortress on the Rhine, Burg Rheinfels was built in 1245 by Count Dieter V of Katzenelnbogen as a base for his toll-collecting operations. Its size and labyrinthine layout are astonishing. Kids (and adults) will love exploring the subterranean tunnels and galleries, accessed by lantern-lit guided English-language tunnel tours; check the website for schedules. Entry fees are cash only. To reach the sprawling ruins, it's a 550m uphill walk; allow 20 minutes. The complex also incorporates a hotel.