The Potocki Palace complex has been an integral part of the Stanislawow city fortress since its reconstruction in the end of XVIth century. The functional and the spatial structure of the complex was formed over two sequential phases: the residence (1680s-1802) and the military hospital (1802-2004). The preserved buildings of the palace period have many layers of different historical development stages and require the application of the restoration methods for its identification. The paper aims to identify the architectural fragments detected on the main façade of the building at the northern wall (building K) and to make an attempt to prove the hypothesis regarding the reconstruction of the complex northern wall and installation the gate in the 1740s during the residence period (1680s - 1802). The existing planning structure was compared to the historical cartography in order to identify the building with the underground chambers below. The findings single out two main construction periods, which correspond to the complex periods of functioning. The first period is the residence of the Potocki family (1680s - 1802), when underground chambers and a portal in the castle wall for access to the terrace probably existed on the same composite axis. The second period - the military hospital (1802 - 2004), during which the building was constructed on the wall, in correspondence to the layout of the underground chambers. The results cast new light on the planning structure of the complex and the city fortress itself and reveal a valuable part of historical heritage. However these architectural fragments require further archaeological research and examination with a ground-penetrating radar. The research was carried out within the framework of the scientific research program of the municipal enterprise Space of Innovative Creativity PALACE, which was created for the scientific research, protection, preservation and revitalization of the complex.