The Theodosian Walls are a series of defensive fortifications built in the 5th century to protect the city of Constantinople. Named after the Byzantine Emperor Theodosius II, these walls were constructed between 413 and 439 AD and included a double line of walls with a moat, towers, and gates, making them one of the most formidable defenses of their time. Stretching approximately 6.5 kilometers, the Theodosian Walls played a crucial role in safeguarding Constantinople from various invasions throughout history. They successfully defended the city against attacks, including the famous siege by the Ottoman Empire in 1453, until they were eventually breached.