With cannons, siege warfare and castle design had to change. Stonethrowing machines were still very effective, but the prestige attached to cannons because of their novelty and their enormous expense made them the ultimate royal armament. These early cannons could be fired only ten or twenty times an hour and had to be cleaned after every shot and regularly cooled. They were effective only at about fifty yards. Cannons required massive earthworks to absorb the shock of firing. Mons Meg, the six-ton cannon still to be seen in Edinburgh castle, was cast in 1449 in Flanders for the duke of Burgundy, who presented it to the Scottish king in 1457. Mons Meg could fire gunstones that weighed 330 pounds nearly two miles, but the cannon was so heavy it took 100 men to move it and then they could move it only at a speed of three miles a day. The Scottish kings used Mons Meg as a siege weapon for the next hundred years, as much for the impressive explosion it produced as for its actual usefulness. After about 1540 the cannon was only used to fire ceremonial salutes from Edinburgh castle walls. In 1681 the barrel burst and could not be repaired. To counter the new offensive weapons, architects created a new system of defense in depth by using low, broad ramparts that were wide enough to endure firing from the enemy and at the same time support their own cannons and teams of gunners. Extremely thick masonry walls were expensive and slow to build, so wide and low earthen ramparts faced with stone became common. Since guns shoot horizontally, the land around the castle walls was cleared to form a space called the glacis. As we have seen at the castle of Angers, existing towers were cut down to the same height as the walls and turned into firing platforms (see Figure 17). This redesign of the towers did not “slight” the castle, but rather made it more effective in the new age of artillery warfare.