In its later guises, the fortress moved the majority of its key installations underground. Fascinating visits, by tour only, walk you through some 500m of the citadel's web of dripping tunnels showing you never-tested 1939 gas-proof bunkers and clever Vauban architectural tricks. Guides help bring the past to life using audiovisual displays and 3D projections on the wall. Be ready for temperatures of around 13°C (55.4°F).
There's always at least one English-language tour per day from April to September (two daily in July and August). Call ahead or check online for the latest schedule.