Fort de la chartreuse, in the middle of Liege on the Mont Cornillon. Before there was a fortress on this location there was a Monastery, where the fortress gets it name from.
The fortress is easily accessible and safe to go in. This is a popular place, we saw many people there and even a professional photoshoot happening.
I need to go back here sometime, as we had not much time and could only explore one and a bit building. This place is pritty big.