The fortress museum occupies two of the original casemates — the arched brick chambers built into the ramparts in the 1680s. You can walk through the underground passages, see the original artillery, and climb onto the walls for a proper view of the star-shaped moat system below. It gives a clear sense of how these defences actually worked: the overlapping fields of fire from the angled bastions meant there was no dead ground for an attacker to shelter in.
The museum also covers the 1572 massacre, when Spanish troops killed most of the town's population during the Eighty Years' War — an event that prompted the Dutch to rebuild Naarden specifically as an impregnable fortress.