The Abbey of Mont St. Michel, which we have already studied under various aspects, has further information to give us with regard to the construction of fortified gateways. In accordance with contemporary usage, the Abbot Pierre Le Roy built a gate-house or _bastille_ (Fig. 163), the entrance of which was guarded by a portcullis and a wide _machicoulis_; he masked this gate-house by a barbican, which was connected north and south with the great stairway leading to the abbey. The northern staircase is rendered specially interesting by the ingenious arrangement of its gates, which opened within the barbican. The apertures were filled by a panel which worked horizontally, on a system necessitated by the exceptional situation of the abbey, where the military, as well as the domestic buildings, were superposed, communicating with each other only by an elaborate series of staircases and inclines. The doors pivoted upon horizontal axes. Resting upon salient jambs in the embrasures of the doorways, they opened in a direction parallel with the slope of the steps, and could be shut at the least alarm, being carried into place by their own weight. They were kept fastened by lateral bolts, the slots of which still exist in the jambs.[68] [68] Ed. Corroyer, _Description de l'Abbaye du Mont St. Michel et de ses Abords_; Paris, 1877. [Illustration: 193. RAMPARTS OF MONT ST. MICHEL. GATEWAY KNOWN AS THE _PORTE DU ROI_] The main gate of the ramparts, which was built between 1415 and 1420, is to the west of the place, in the curtain flanked by the tower known as the _Tour du Roi_. This gate and the lateral postern gave access to the town, their drawbridges forming a passage across the moat when lowered, and when raised, an initial barrier to assailants. Above the gates was the warder's lodging, beneath which the vaulted passage and the postern communicated directly with an outer guard-room in the ground-floor of the _Tour du Roi_. In addition to the first barrier, formed by the raised platform of the drawbridge, the main entrance was secured by double doors, and by an iron portcullis, which still remains in its lateral grooves. The great arch is crowned by a tympanum, on which the united arms of the king, the abbey, and the town were carved. The works designed for the defence of rivers flowing through fortified towns, or of the inlets of harbours, are closely allied to the military architecture of gates. At Troyes the river arches in the town ramparts were guarded by gratings or portcullises of iron. At Paris the passage of the Seine was barred by chains stretched across the river from wall to wall, and upheld in the middle of the stream by piles or firmly anchored boats. At Angers the walls of the town abutted on two towers known as the _Haute Chaîne_ and the _Basse Chaîne_ (the Higher and Lower Chains), containing windlasses for the chains, which at night were stretched across the Maine at its passage through the _enceinte_.