 Complete the following phrase:  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 