Other keeps of equal interest in point of situation, plan, or details of construction are:--Ste. Suzanne, Nogent-le-Rotrou, Broue, L'Islot, Tonnay-Boutonne, Pons, Chamboy, Montbazon, Lavardin, Montrichard, and Huriet in the Bourbonnais. All these, in common with those first described, are square or rectangular on plan. From the end of the twelfth century onwards the cylindrical form predominates in the plan of keeps and towers. On the whole, it offered the best resistance to the mediæval assailant. The convex surface was of equal strength all round, and as we have seen in the preceding chapter, the circular trace for towers gave the garrison the best chance of defending their bases from the curtain, and of opposing the work of sappers and miners. [Illustration: 180. PROVINS CASTLE. KEEP] The great advance made in architecture by the general adoption of an expedient so simple and easy of execution as the vault on intersecting arches manifested itself very strongly in military structures. The heavy wooden floors of the earlier keeps, which were so apt to catch fire, were replaced by less ponderous vaults, binding the circular walls firmly together, and forming a flooring for the various stories less unsteady and infinitely more durable than the huge beams and joists of earlier days. A further improvement was the pointed roof, round on plan, now generally adopted as better calculated to withstand projectiles or combustibles which shattered the angles of the roof in the old square towers, and set fire to the timbers. The form of keeps, however, varied considerably throughout the twelfth century. At Houdan the keep is a great tower strengthened by four turrets; at Étampes it is composed of four clustered towers, forming a quatrefoil on plan; the vaulted stories are marked by many curious features, among others a deep well, the opening of which is in the second floor. Some historians date this building from the eleventh century; there are indications, however, in the details of the architecture and sculptures, which point to the early part of the reign of Philip Augustus. The keep of Provins, which belongs to the twelfth century, has certain very original features. It rises from a solid mound of masonry, and has a circular _enceinte_. The base of the keep itself is square, and is flanked at each angle by a turret. An octagonal tower surmounts the square base, and is connected with the flanking turrets by flying buttresses. The keep of Gisors is also octagonal in form, one of its octagons being at a tangent to the circular _enceinte_ which crowns the feudal _motte_ or mound. It was built in the twelfth century, and was considerably augmented by the line of walls and square towers which Philip Augustus drew round the mound.