The numerous and mysterious tunnels inside the bastions were constructed simultaneously with the building of bastions in the seventeenth and eighteenth centuries.
The earliest tunnels were obviously constructed in the 1630s when the earth mounds, the old Wismar Ravelin above all, were heaped up in front of Tõnismägi (St Anthony’s Hill). Construction became more intensive beginning from 1672 when the Swedish Bastion and the Ingermanland Semi-Bastion were built. According to the fortification plan confirmed by the king in 1686, the Swedish Bastion, the Ingermanland Bastion, the Wismar Ravelin and the De la Gardie Redoubt in front of the latter were built and they had been completed by 1710.
The Ingermanland and Swedish Bastions are typical pentagonal earth mounds supported by stone escarp walls. The purpose of the bastions was to enable firing upon the attacking enemy on both flanks. The guns were lined up on top of the bastion. In between the bastions the triangular Wismar Ravelin was situated, in front of the fortifications was the moat with its long wooden bridges.
The hidden tunnels were constructed inside the bastions in order to shelter the men, ammunition and supplies but especially to hide their relocation from the enemy. The passages also served as observation posts to spy the enemy’s activities in mining explosives.
The tunnels were made of limestone. They were 1.5 m wide and 2.5-3 m high, as a rule, covered by barrel vaults. The walls were up to four meters thick. The passages along the escarp walls were provided with 2.5-meter-wide chambers – the so-called casemates. The ceilings of the passages were provided with ventilation holes, different levels were connected by stairs. The total length of the passages is hundreds of meters and 470 m of them are open to the visitors.
Tunnels connect both bastions and the former Wismar Ravelin that does nor exist any longer. In case of the latter, the passages show us the borders of the ravelin as it used to be. The bastions never witnessed more serious campaigns and thus the tunnels were never actively used either.
The tunnels were partially used as storerooms in the eighteenth century already. In 1857 the earth bastions were erased from the list of fortifications and after that the Municipality turned them into parks.
Beginning from 1936 the passages wereby and by turned into air-raid shelters. In the years of Soviet occupation they were provided with separate electricity and ventilation systems, running water and telephone communications. The network became an underground township with bedrooms and toilets included. In 1976-77 the youth movement Kodulinn did some cleaning in the passages. In the 1980s the passages under the Ingermanland Bastion (Harjumägi) were used as a store for sculptures belonging to the Art Foundation but the danger of thieves forced the authorities to move the sculptures elsewhere. Since then the tunnels have been empty. Beginning from the 1990s some homeless people found shelter in the deserted tunnels.