Luxembourg is famous for being the "Gibraltar of the North", which is due to its extensive fortifications. Basically, it had more reinforced bunkers and fortresses than the entire Maginot line. Although they signed a treaty in the 19th century to make Luxembourg neutral and dismantle the catacombs, they figured out they really couldn't without basically blowing up the entire city. They basically settled on just blocking the critical junction points in a lot of areas, so some of the catacombs still survive.
The funniest part is that after spending nine months in Luxembourg, I never saw them until this weekend. Joe says he didn't either, and he thinks it was because it was too expensive at the time (2 euros). Amazing that I lived on a student budget like that.