The medieval moats that originally surrounded the Špilberk fortress were abolished in 1742 in connection with the building of the casemates. The moat on the southern and northern sides was modified and subsequently spanned by two-storey massive brick vaults. The eastern and western moat was thus connected only by the waste channel passing through the lowest floor of the southern casemates. It drained water from the western moat to the mouth of the waste tunnel in the eastern moat. The ‘Rat Canal’ was connected with many legends. One legend depicts in particular the brutal torture of prisoners, who were lowered into the canal and left to predatory rodents. The number of rats in these places was apparently no exaggeration, but the truth of the legend ends there. Neither this nor any other form of torture ever took place in Špilberk prison.

When Špilberk was converted to a Baroque fortress, it was necessary to drain the remaining moats. The original waste tunnel was probably only a short canal passing through the fortification wall, leading to the bare slope of Špilberk Hill. After 1742, however, the waste channel was dug through the rock wall and led to Pekařská Street. The construction is really very interesting. The solid rock mass had to be enhanced in some parts by bricks or stone. The gallery varies widely in size and gradient. Even the direction is not straight and the corridors snake to the right and left.

That this stone gallery emerged in the mid-18th century is only hypothetical. Whether it was created much earlier and could therefore also have served as a secret or emergency route from the besieged castle is not yet possible to prove. However, hints of other corridors and impassable branches only confirm the fact that Špilberk still hides a number of historical and hitherto unknown underground spaces.