The Diyarbakr City Walls (DCW), which were recently added to UNESCO's World Heritage List, are among the largest and one of the most impressive monuments from ancient times. The history of the DCW stretches back more than 4000 years, therefore making the extant City Walls a combination and reflection of influences of the various civilizations that settled in the region. Basalts having such different textural properties as massive and vesicular were employed as the principal material in the construction of the DCW. Like many other historical structures, the DCW are suffering from stone deterioration. A large variety of weathering forms can be observed on the basalts used in different sections of the DCW. The behavior of the basalt under different cyclic environmental conditions is not known. In this study, deterioration mechanisms of the basalts were investigated with field and laboratory studies. It is found that iddingsite is a common weathering product developed through the crystal boundaries of olivine. Accelerated weathering tests show that salt crystallization is the most effective deterioration mechanism. The field and laboratory studies indicate that both massive and vesicular basalts are durable; however, the massive basalts are more durable than the vesicular ones. It is also observed that although chemical processes slightly trigger the deterioration mechanisms of the rocks, most of the weathering forms on the DCW are controlled by the physical factors affecting the parameters of the basalts such as porosity, water absorption and uniaxial compressive strength.