In Iran, castles are generally classified into two main categories: mountain castles and plain (lowland) castles. Mountain castles were constructed along passageways, mountain ridges, beside valleys, or near rivers, and their primary purpose was to ensure security and defense. In northwestern Iran, due to its distinctive topographic characteristics, thousands of mountain castles exist. These fortifications were built from the fourth millennium BCE, coinciding with the emergence of political and social instability, and their construction continued until the Qajar period. The design and layout of mountain castles were strongly influenced by the surrounding topography, and local stone materials from the same region were commonly used in their construction. Because of their mountainous geographical location and the use of indigenous materials, these castles are highly vulnerable to deterioration. The present article adopts a descriptive–analytical method, relying on both field studies and library-based research, to examine the natural factors (such as precipitation, freeze–thaw cycles, soil erosion) as well as human factors that contribute to the destruction of these monuments. The two main research questions of this study are as follows:

1. What materials were most commonly used in the construction of mountain castles?

2. Which factors have the greatest impact on the deterioration of these structures?

Based on the findings of this study, it was determined that stone and sarooj mortar were the most widely used materials in the construction of mountain castles in northwestern Iran. Furthermore, factors such as erosion, earthquakes, human activities, and environmental conditions have the greatest impact on the degradation of these historical structures.