Most of the terms used for throwing machines were vague, and could refer to different engines, all of which went through changes and developments over the period. Among the most common were the ballista, mangonel and trebuchet. The ballista was similar in form to a crossbow, though much larger, and used a string-winding mechanism to fire a missile or bolt placed in a groove. Other giant crossbows were used throughout the period, and an "espringal", based on the ballista, which threw large bolts, was developed in the 13th century. Torsion-powered arrow firers had been used from 400 BC, and were adapted for stones. A mangonel had a wooden spoon-shaped arm, to hold a stone or other projectile, which was manipulated under tension from a twisted rope. The trebuchet was an advanced development of the 12th or 13th century, which used a counter-weight to power the throwing arm, and was the major siege engine until the cannon became widespread. Forces attacking a castle or other strong fortification sometimes sought to undermine the foundations by digging "mines" or tunnels underneath them. Usually, such mining or digging machinery was protected by a "tortoise" (also called a "cat", "sow", or "owl"): a covered shed on wheels, which shielded the miners from missile attack. As the tunnels were constructed, they were generally supported by wooden beams and posts. Once the mine had been finished, the internal space was filled with combustibles, such as brushwood, firewood, resin, and other incendiary substances; once ignited, these would burn the supporting props, causing the mine to collapse, bringing down with it the structures lying above. From the 15th century, gunpowder was also used, although the aim remained to burn the props. Defenders might sometimes dig counter-tunnels in order to reach the enemy's mines and launch an attack; frequently thermal weapons were used to drive the besiegers from the tunnels. Rather than undermining a structure, some besiegers used borers to drill holes in the outer walls in an effort to destroy them; such methods were more effective than rams on brick walls (which tended to absorb the shocks from the ram). Borers differed in size and mechanism, but a typical machine was made from a log of wood, tipped with iron and supported and driven by windlasses or ropes. Once a series of holes had been bored along the length of a wall, the holes were typically filled with rods of dry wood, saturated with sulfur or pitch and then ignited.