Since the degradation of building stones is a process that cannot be stopped, the three mainknown types of rock alteration, chemical, physical, and biological affected the Historical Wall of themedieval city Cluj-Napoca, too. The crusts’ formation on the wall’s surface favours the occurrence ofgypsum, whose volumetric expansion leads to the generation of microfissures. The most frequent type offilms noticed is represented by the black-stained crust. The crusts are mixture of gypsum, fibrous calcite,isotropic material and silt quartz, small carbonate crystals, and clay indicating the constant present ofmoisture, pollutants, and black particles from industrial dust, ultra-fine soil components, and also somepossible restoration attempts. The dissolutions inside, especially near the surface of the rock formed largeamounts of microporosity. Large vugs and cracks were also formed by the breakout of the bioclasts. Tounderstand the mechanism of their formation some microscopic analyses, SEM pictures and XRDanalyses have been used.