Some 12km northwest of Mechelen, this haunting, moated prison-fort was built in 1906. However, its use as a notorious Nazi internment camp in WWII is the main focus of the audio-guided visit, with over two hours of detail including harrowing personal accounts: around 3500 victims were subjected to its torture rooms, cells and dark, dank corridors. An extra exhibition also looks at later war-crimes trials.
Note that much of the visit is outside so dress appropriately. The fort is just off the A12 Antwerp–Brussels highway at junction 7. The nearest train station is Willebroek, on the Leuven–Mechelen–St-Niklaas line, which is a 1.5km walk northeast of the fort (or a quick hop by bus 260 from nearby Schalkstraat). On weekdays you can get bus 460 direct to Brussels’ Gare du Nord (50 minutes) at 33 minutes past the hour from outside the fort.