What other natural and constructed features helped defend Constantinople?

Other natural and constructed features that helped defend Constantinople included the Theodosian Walls, a series of walls and towers built in the 5th century; the Golden Gate, a monumental gate built in the 6th century; and the Sea Walls, a series of walls and towers built in the 7th century. Additionally, the city was surrounded by a moat, and the harbor was protected by a chain that could be raised to prevent enemy ships from entering.