The Cave and castle ruins were definitely worth a visit. There is a tour every half hour in Dutch and if you are English speaking they give you a print out of all the important facts and commentary on the interesting things to see. You have to take the tour otherwise getting lost would be certain. At the end of the tour you come out in the castle ruins and can explore those with good views over Falkenberg.
Well worth while. Adults cost 9 euros each which includes the 50 min tour and you can roam the ruins at leisure afterwards.