Cambrai was for many years on the boundary between the Kingdom of France and the Holy Roman Empire. As such, sound fortifications were desirable and an immense citadel was built in the 16th century by Charles V. The citadel was dismantled in the late 19th century but remarkably many kilometres of underground passages and counter-mine galleries remain. Only a few remnants survive above ground, including the fine Royal Gate (Porte Royale).
Sub Brit was fortunate in being given special access to this extensive underground network in May 2002. Today a small portion of the tunnels are opened on a regular basis for pre-booked tours by the tourist office.