The chain!

The Byzantine Empire had walls built along the shoreline to protect the city of Constantinople from naval attacks. At the entrance to the Horn on the northern side, a large chain was pulled across from Constantinople to the old Tower of Galata to prevent unwanted ships from entering. Known among the Byzantines as the Megàlos Pyrgos (meaning "Great Tower" in Greek), this tower was largely destroyed by the Latin Crusaders during the Fourth Crusade in 1204. In 1348, the Genoese built a new tower nearby which they called Christea Turris (Tower of Christ), now called Galata Tower. There were three notable times when the chain across the Horn was either broken or circumvented. In the 10th century the Kievan Rus' dragged their longships out of the Bosphorus, around Galata, and relaunched them in the Horn; the Byzantines defeated them with Greek fire. In 1204, during the Fourth Crusade, Venetian ships were able to break the chain with a ram. In 1453, Ottoman Sultan Mehmed II, having failed in his attempt to break the chain with brute force, instead used the same tactic as the Rus'; towing his ships across Galata over greased logs and into the estuary.