This research addresses the sandstone masonry of Durham Castle, a World Heritage site located in Durham, England. The study encompasses a focus on its current condition, deterioration mechanisms, and in particular, alveolar erosion, and the performance of previous repair techniques. The stone and weathering observed at the castle appear to be consistent across the entire site, regardless of age or location and are representative of other buildings in the area. The obvious diagnosis therefore appears to be related to the geo-chemical nature of the stone more than any other single factor. Alveolar erosion is particularly evident and poses the greatest risk given its resultant loss of stone and unit volume, leading to visual disfigurement and structural instability. Because of this long-lived problem, composite mortar repairs and stone replacement have frequently been performed on the castle. This study researches this current deterioration mechanism through literature reviews, archival research, on-site survey and investigation, and material analysis including thin section petrography, soluble salt and clay identification, porosity, and water absorption/desorption. This study concludes with possible sources of alveolar erosion at Durham Castle and conservation recommendations to maximize the retention of original stone through both preventive and remedial treatments before replacement in kind becomes necessary.