Constantinople, for the most part, was surrounded by natural barriers. Around Constantinople were seven hills. In 328 A.D, Byzantium out up a defensive wall around where there were not natural barriers. Having the entire area protected with walls made it difficult for people to invade the area, and it only had one way around the walls. On the western side of the city was a route inside, and that was the source for many of the attempted attacks that Constantinople continuously won.
Constantinople was surrounded by seas on three sides, to the north was the Golden Horn, to the east was the strait Bosporus, and to the south was Sea Marmara. Bosporus was rarely calm and that made it difficult to attack through there. It had very strong currents that could sweep hostile ships away. The Golden Horn, during time of war, sheltered Constantinople's ships. Also, the Golden Horn made it possible for them to receive supplies by sea.
These natural barriers made it nearly impossible for anybody to successfully invade Constantinople until the Ottomans in 1453.
Constantinople was surrounded by seas on three sides, to the north was the Golden Horn, to the east was the strait Bosporus, and to the south was Sea Marmara. Bosporus was rarely calm and that made it difficult to attack through there. It had very strong currents that could sweep hostile ships away. The Golden Horn, during time of war, sheltered Constantinople's ships. Also, the Golden Horn made it possible for them to receive supplies by sea.
These natural barriers made it nearly impossible for anybody to successfully invade Constantinople until the Ottomans in 1453.