Firstly you must visit this place do both the caves and the fort as a combined ticket allowing 3 hours if you start on the 12:30 tour. You get to go underground through the tunnels guided by touch in a way modern society doesn't seem to let you do any more so makes you feel like you are exploring. All in all the best thing to see in Maastricht, made better my our amazing English speaking guide Justyma (sorry for the spelling), who added humour and a great deal of knowledge to the tour.