This years expedition was to the Limburg Capital of Maastricht were we visited the St Peter Fortress. In 1672 the St Peter Hill was used by the French to bombard the city and near the city using the hill's cave system (the rock is used as a building material), so the Estates General ordered a new fortress to be build to protect the city, which went rather well. In 1701 the fortress with a great view on the surrounding landscape and the river Meuse was finished.
It is also possible to visit the cave system itself but it is not possible anymore to go from the caves to the fortress, as some of the corridors have collapsed.
Maastricht has a lot to offer, some of the old city walls are standing there as well, as the first picture shows.