warfare. Cannons were first used in Song dynasty China during the early 13th century, but did not become significant weapons for another 150 years or so. In early decades, cannons could do little against strong castles and fortresses, providing little more than smoke and fire. By the 16th century, however, they were an essential and regularized part of any campaigning army, or castle's defences. The greatest advantage of cannons over other siege weapons was the ability to fire a heavier projectile, farther, faster, and more often than previous weapons. They could also fire projectiles in a straight line, so that they could destroy the bases of high walls. Thus, 'old fashioned' walls – that is, high and, relatively, thin – were excellent targets, and, over time, easily demolished. In 1453, the Theodosian Walls of Constantinople, the capital of the Roman Empire, were broken through in just six weeks by the 62 cannons of Mehmed II's army, although in the end the conquest was a long and extremely difficult siege with heavy Ottoman casualties due to the repeated attempts at taking the city by assault. However, new fortifications, designed to withstand gunpowder weapons, were soon constructed throughout Europe. During the Renaissance and the early modern period, siege warfare continued to dominate the conduct of the European wars. Once siege guns were developed, the techniques for assaulting a town or fortress became well known and ritualized. The attacking army would surround a town. Then the town would be asked to surrender. If they did not comply, the besieging army would surround the town with temporary fortifications to stop sallies from the stronghold or relief getting in. The attackers would next build a length of trenches parallel to the defences (these are known as the "First parallel") and just out of range of the defending artillery. They would dig a trench (known as a Forward) towards the town in a zigzag pattern so that it could not be enfiladed by defending fire. Once they were within artillery range, they would dig another parallel (the Second Parallel) trench and fortify it with gun emplacements. This technique is commonly called entrenchment. If necessary, using the first artillery fire for cover, the forces conducting the siege would repeat the process until they placed their guns close enough to be laid (aimed) accurately to make a breach in the fortifications. In order to allow the forlorn hope