In 1453 Mehmed commenced the siege of Constantinople with an army between 80,000 and 200,000 troops, an artillery train of over seventy large field pieces, and a navy of 320 vessels, the bulk of them transports and storeships. The city was surrounded by sea and land; the fleet at the entrance of the Bosphorus stretched from shore to shore in the form of a crescent, to intercept or repel any assistance for Constantinople from the sea. In early April, the Siege of Constantinople began. At first, the city's walls held off the Turks, even though Mehmed's army used the new bombard designed by Orban, a giant cannon similar to the Dardanelles Gun. The harbor of the Golden Horn was blocked by a boom chain and defended by twenty-eight warships.