This paper focuses on the underground military galleries and countermine system of the Petrovaradin Fortress near Novi Sad (Serbia). The Petrovaradin Fortress was presented within the working group Underground Built Heritage Reuse and Valorisation Strategies on the COST action Underground4value as a good example of tourist valorised underground cultural heritage in Serbia. The goal of the paper is to consider as widely as possible the options that could contribute to the sustainability of these galleries. Numerous field observations, complex research of the literature as well as other available sources, and interviews with visitors to the fortress were necessary for this goal to be achieved. The paper has special historical and practical significance. On the one hand, it represents a record in time concerning the condition of the Petrovaradin Fortress. The practical significance is reflected in the scientific contribution based on the critical analysis of the results obtained from authorities on the Fortress and visitors, as well as the ideas that the authors conceived or adapted from similar sites in the world, assessing that these ideas can be applied to the Petrovaradin Fortress. The results of the paper explain the aspects of ecological, economic, and social sustainability of the Petrovaradin Fortress, and indicate the need for it to be supported by various institutions.