When entering the Botanic Gardens, the first thing you see is the Rock Garden. At its highest point, it is approximately 12 metres above the level of the fort moat. In terms of its wealth of different species and surface area, it is one of Europe's largest rock gardens.
Construction started on it in 1963, on top of the fort bunkers. Of course, the height difference of these bunkers proved particularly advantageous. However, 2,100 tonnes of rock still needed to be brought in from the Ardennes to create and 'dress' the Rock Garden. Because the stones suffer from occasional subsidence or part of the Rock Garden may need to be replanted, additional blocks of rock are regularly needed.