The underground tunnels system under the fortress and hills Kaštel consists of a lower complex of underground tunnels at the city level, the so-called Zerostrasse, and 11 m above them an upper complex of underground tunnels – the mezzanine. The fortress Kaštel is located 11 m above the mezzanine tunnels.
Pula underground tunnels were created during the Austro-Hungarian rule, just before the First World War for the needs of sheltering people in case of direct aerial attacks on the city. The Venetian fortress on top of the central city hill Kaštel (Hasfenkastell) served as warehouse, barracks and watchtower under Austrian rule. After the First World War, Italian authorities continued using Pula underground tunnels, widened and expanded them. After the Second World War, they were used as shelter for civilians and for the needs of medical corps. The basic idea of revitalizing the underground tunnels system is connecting the tunnels with the fortress Kaštel in one unit and facilitating access to the fortress from the level of city streets at the foot of Kaštel hill with an elevator. The primary request of the employer, which was for the last station of the elevator to be inside the Historical and Maritime Museum of Istria, was taken into account in the selection of elevator location.
The elevator connects all three levels, with three stations. The first station is in tunnels ZeroStrasse and it is located in the immediate vicinity of the central square that spreads in 4 channel arms. The reconstruction of the existing space with the access hall Zerostrasse was executed, as well as the access hall of the mezzanine (the second station). The third elevator station is inside the fortress courtyard and is the only visible element of the elevator structure. With the aim of fitting the new element as best as possible in the historic unit of the fortress Kaštel it was only appropriate to select glass from the esthetic, safety and functional standpoint.
The Zerostrasse tunnel walls were carved in bedrock, and it was necessary to preserve them as much as possible. Natural concrete and smooth lining were used in reconstruction. The hydrant network installations and point water drainage were realized within floor surfaces. In the central part of the tunnel, works were realized on concrete benches, and new concrete floors were executed, as well as rehabilitation of benches in all other tunnels. With the approval of the conservationist, the old metal entrance doors were removed on all entrances, and new doors were realized with lattice leaves for air passage.