The Bastionder museum is built into the Bastion Oranje, one of the best-preserved sections of Den Bosch's 16th-century Spanish fortifications. The bastion is a massive underground structure of arched brick chambers and tunnels designed to protect the city from artillery bombardment — and remarkably, most of it survives intact.
The museum tells the story of Den Bosch's fortifications and the dramatic 1629 siege that ended Spanish rule in the city. The underground spaces themselves are the main attraction: low brick vaults, gunpowder stores, and sally port tunnels give you a vivid sense of what it meant to defend and besiege a fortified city in the 17th century. A good combination with a walk along the remaining city walls above.