developments in military architecture. Speaking generally, we may say that just as there was comparatively little change in armour during the 12th century until the end of Henry II.'s reign, so there was comparatively little change in military architecture during the same period. But great changes took place towards the end of the 12th century. One of these changes was a great improvement in missile engines; the trébuchet was one of the most important of these. It could throw much heavier stones than the largest catapult, and could take a more accurate aim.[1174] These new engines were useful for defence as well as attack, and this affected the architecture of castles, because flat roofs covered with lead, on which machines could be placed, were now substituted for the former sloping roofs.[1175] There are several payments for lead for roofing castles in the _Pipe Rolls_ of Henry II., the earliest being in 1166. In the reigns of John and Henry III. the mention of lead for roofing becomes much more frequent.[1176] Hitherto, in the defence of keeps, reliance had mainly been placed upon their passive strength, though not so entirely as has been commonly assumed, since it was always the practice to shoot with arrows from the battlements round the roof of the tower. But not only was the fighting strength of the keep increased by the trébuchet, but the introduction of the crossbow gave it a defensive arm of the greatest importance. The crossbow had been known to the Romans, and was used in the early part of the 12th century, but it was forbidden by the second Lateran Council in 1139 as a weapon hateful to God.[1177] This prohibition seems actually to have been effective, as William the Breton says expressly that the crossbow was unknown to the French before the wars of Richard I. and Philip Augustus.[1178] Richard learned the use of it in the third crusade.[1179] But to use the crossbow in the defence of buildings it was necessary to construct special loopholes for shooting, splayed downwards externally, so that it was possible to aim from them. Up till this time the loopholes of castles had been purely for light and not for shooting; anyone may see that it is impossible to take aim through an immensely thick wall unless there is a downward splay to increase the field of vision. William the Breton tells us that Richard built windows for crossbows to his towers, and this is the first mention we have of them.[1180]