susceptible to fire, since they contained many combustible materials. In 1139, Henry de Tracy forced the surrender of Torrington Castle by the simple expedient of tossing lighted torches through the keep's loopholes. Stone was also susceptible to intense heat, which would cause it to crack and collapse. Byzantine sources recorded the demolition of stone structures caused by placing clay pots of burning charcoal at the base of walls moistened with vinegar or urine, and the 6th century treatise by an engineer in Justinian's army includes the lighting of fires beneath the walls amongst its instructions for sieges. Stone castles sometimes offered other inflammatory targets. During the Crusades, Muslim defenders frequently hung bundles of straw against their walls as buffers against stones and rams; in turn, the Crusader archers would set these alight with fire arrows. Defense against thermal attack Defense from thermal weapons and fire attacks was usually water or other liquids such as urine; hides were soaked and draped over vulnerable wooden hoardings and siege engines, vats and barrels of liquid were collected and stored by defenders and attackers. Hides were hung in an overlapping manner so that any water would run down the whole structure to extinguish flames. Some thermal weapons (such as quicklime or oil) could not be extinguished or eased by water, in which case sand or earth could be used. Wooden structures were frequently soaked in alum to increase their fire resistance. The Romans covered their tortoises (mobile siege sheds) with raw hides packed with vinegar-soaked seaweed or chaff, to serve as protection against regular and incendiary missiles. Throughout the period, sacks or nets might be filled with vinegar-moistened chaff, seaweed or wet moss and hung on the exterior. The wooden siege engines of the Crusaders were vulnerable to attack from the Byzantine and Muslim fire-weapons, so the troops inside siege towers kept stores of water and vinegar. During the High Middle Ages, the majority of Poland's castles were still made of wood, so uncut stone was frequently added to the front to improve their fire defences. Both attackers and defenders needed to be prepared for incendiary and thermal attack. When the Athenians besieged Syracuse in 416 BC they lost many siege engines to fire. The Syracusan ruler Dionysius I must have taken note of this success, for when he laid siege to Motya in 398 BC he organised special fire "brigades", who successfully doused