A network of underground passages built into Maastricht's medieval city walls — designed to move troops and supplies between fortifications without exposure to enemy fire, and to place cannon at strategic points along the walls. Maastricht was besieged more than twenty times over its history, and the casemates represent the city's cumulative architectural response to that pressure, built up from the 16th to 18th centuries.
Guided tours run through sections of the tunnels near the Helpoort and the Sint Pietersberg area, passing powder stores, garrison chambers, and gun positions. Less visited than the more famous Sint Pietersberg caves, but the casemates give a different perspective — the experience of the medieval defender rather than the quarry worker.