From this time defensive loopholes become common in castles, and take various fanciful forms, as well as the commoner ones of the circle, square, or triangle at the base of the loop. The _cross loophole_, which does not appear till the latter quarter of the 13th century, is explained by Viollet le Duc as an ingenious way of allowing three or four archers to fire in a volley.[1181] But up to the present time very little study has been given to this subject, and we must be content to leave the question for future observation to settle.[1182] The crossbowmen not only required splayed loopholes, but also niches, large enough to accommodate at least three men, so that a continuous discharge of darts (quarrells) might be kept up. Any defensive loop which really means work will have a niche like this behind it. These niches had the defect of seriously weakening the wall. Another innovation introduced by Richard I. was that of stone machicolations, or _hurdicia_.[1183] Whether wooden galleries round the tops of walls, with holes for dropping down stones, boiling-water, or pitch on the heads of the besiegers had not been used from the earliest times, is regarded by Köhler as extremely doubtful.[1184] They were certainly used by the Romans, and may even be seen clearly figured on the Assyrian monuments. In the Bayeux Tapestry, the picture of Bayeux Castle shows the stockade on top of the motte crested with something extremely like hurdicia. Yet the writer has found no authentic mention of them before the end of the 12th century.[1185] The stone machicolations built by Richard round his keep of Chateau Gaillard are of an unusual type, which was only rarely imitated.[1186] But from this time wooden hurdicia became universal, to judge from the numerous orders for timber for _hoarding_ castles and town walls in the _Close Rolls_ of the first half of the 13th century. Towards the middle of the 13th century stone brackets for the support of wooden hurdicia began to be used; they may still be seen in the great keep of Coucy, which was begun in 1230. But machicolations entirely of stone, supported on double or triple rows of brackets, do not become common till the 14th century.[1187]