Great Chain

The Bosphorus Chain was a giant chain at the entrance of the Golden Horn pulled from the Galata Tower, preventing unwanted ships getting in the Golden horn.

History
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 into the estuary over greased logs