Owing to the developments in computer aided structural analysis methods in the last two decades, it has been possible to realize structural analysis of historic masonry structures in a very detailed and precise way. Besides the structural analysis package programs, special software have also been developed based on the characteristics of the case in question. While concentrating on the advanced methods of mathematics and mechanics, it might be possible to overlook the main point, which is the relationship between structural stability and the natural events. In this study, structural stability issues and the consequent structural damages on the fortification walls of Sinop Castle under the effects of sea waves have been explored. Sinop Castle is known to have been built by the immigrants who settled in the region at eighth century BC. The fortification walls that were destroyed by the attacks of Cimmerians in seventh century BC were fixed during the reign of King Mithridat IV and they were expanded to the present day limits. Having undergone constant maintenance during the periods of Romans and Byzantines, the castle was invaded the Seljouks between 1214 and 1261. During this period, an inner castle was built to fortify the defense of the repaired castle. The fortification walls of the Sinop Castle completely surround the narrowest pass of the peninsula. The northern walls are 1800 m, northern walls are 400 m, eastern walls are 500 m and the western walls are 273 m long. The walls reach a height of 30 m with a thickness that varies between 3 and 8 m. The northern walls have sustained excessive deterioration due to the sea waves and only the Kumkapi and Lonca gates have survived. In this study, stability analysis has been performed by using the finite element method on a partial analytical model of the northern walls of the Sinop Castle. The erosion caused by the waves at the bottom of the walls have been taken into consideration in a gradual manner to identify the origin of the damage, which has been caused by the excessive stress due to balance degradation in the massive walls; and consequently, appropriate strengthening methods have been proposed.