Located high above the river Meuse, Namur citadel is one of the most impressive military complexes in Europe.
You can wander above ground through a baffling maze of earthworks, bastions and barracks, but the most impressive fortifications lie deep below ground.
Most of the tunnels are closed off, but there is one place where you can walk through a secret passage inside a seventeenth century bastion.
The entrance signed Galeries de Boileau leads into a damp, dripping passage lined with narrow loophole windows, each with a horizontal iron bar to rest the gun.
Derek Blyth’s hidden secret of the day: Derek Blyth is the author of the bestselling “The 500 Hidden Secrets of Belgium”. He picks out one of his favourite hidden secrets for The Brussels Times every day.