An 18th-century fortress on top of the Sint Pietersberg hill, built to defend Maastricht from the south. The fort saw action only once — during the French siege of 1748 — and the cannonball scars are still visible in the walls. The casemates (underground gun chambers) are well preserved and can be explored on a guided tour.
The real reward is the location: the top of the hill offers wide views over the Meuse valley and into Belgium and Germany — Maastricht sits at the point where three countries nearly meet. The fort and the caves below it can be combined in a half-day visit. The walk up from the city along the river takes about 30 minutes and passes through pleasant parkland.