Cârţa Cistercian Monastery, built in the 13th century during the colonization of Transylvania by Saxons, is the oldest preserved monument in Romania built in the Cistercian style. Tertiary (Sarmatian) calcareous sandstones from the southern part of the Transylvanian Basin were used for construction. The sandstones, rich in quartz, feldspars and rock fragments, are feldspathic litharenites with carbonate cement and bioclasts having a very low porosity. These are not suitable as building stones due to the influence of the CaCO₃ and clay minerals content as opposed to the compact framework of the stone that does not allow expansions; micro fissures and dissolutions occur due to the chemical weathering. The low ultrasonic velocity proves granular disintegration of the sandstones, which presents a medium to high anisotropy. The weathering processes, macroscopically analyzed, belong to Discoloration/Deposits, Detachments, Loss of stone material, and Fissures/Deformations groups. The microscopic analysis evidenced specific weathering processes: fractures parallel to the surface of the stone, fissures running around different types of grains, radial growth of gypsum crystals, formation of crusts in varying generations, and lichens colonization.