The greatest architectural change witnessed at the end of the 12th century was the victory of the round keep over the square. Round towers were built by the Romans as mural towers, but the universal type of mediæval keep appears to have been the square or oblong, until towards the end of the 12th century.[1188] The polygonal keep was probably a transitional form; we have seen that Henry II.'s polygonal keep at Orford was begun as early as 1165. Many experiments seem to have been made at the end of the 12th century, such as the addition of a stone prow to the weakest side of a keep, to enable it better to resist showers of missiles. Richard I.'s keep at Chateau Gaillard is a round keep with a solid prow of this kind. Five-sided keeps are said to be not uncommon on the left bank of the Rhine and in Nassau; this type was simply the addition of a prow to a square keep. The only English instance known to the writer is that of Mitford, Northumberland, but this is merely a five-sided keep, the prow is not solid, as at Chateau Gaillard. The castle of Étampes, whose plan is a quatrefoil, is assigned by French archæologists to this period of experiment.[1189] But the round keep was eventually the type preferred. Philip II. thought it necessary to add a round keep to the castle of Gisors, after he had taken it from John, and he adopted the round keep for all his new castles, of which the Louvre was one.[1190] Along with the round keep, ground entrances became common.[1191] Viollet le Duc states that when the French soldiers broke into the inner ward at Chateau Gaillard the defenders had no time to escape into the keep by the narrow stair which led to the first floor, and consequently this proud tower was surrendered without a blow; and that this event so impressed on Philip's mind the danger of difficult entrances that he abandoned the old fashion. This may be true, but it is a pure guess of Le Duc's, as there is nothing whatever to justify it in William the Breton's circumstantial narrative. It is, however, certain that Philip adopted the ground entrance to all his keeps. In England we find ground entrances to many round keeps of the 13th century, as at Pembroke; but the older fashion was sometimes retained; Conisburgh, one of the finest keeps in England, has its entrance on the first floor.[1192]