An extensive complex of interconnecting passages on 2-3 levels, stretching at least 17km, and built from the 13th century onwards. Cellarage some 8-12m below ground level was complemented by drainage tunnels, sewerage and wells. All of the houses on the square originally had their own entrances to the underground, which served for food storage and for shelter in times of trouble; today, access can be gained from Perlová ul 4.