When the fortress was under siege, it was common for enemies to dig tunnels under the defensive outwork in attempt to blow it up. This forced the need for a countermine passage system for the defenders to blow up both the artillery positions and the potential tunnels of the besieging enemy. The total length of these corridors is around 40 km. Since they were never used, they are preserved in their original form and a part of the labyrinth is open to visitors.