The difficulties of passage were increased by the multiplication of portcullises, the windlasses of which were worked from different stories of the tower, so as to prevent collusion between different parties of the garrison, which was often composed largely of mercenaries. In the gate-house of Carcassonne the first portcullis was raised or lowered by means of chains and counter-weights worked from a windlass on the second floor; the second portcullis was worked in like manner from the first floor, in a place entirely cut off from communication with that above, to which access could only be obtained by a wooden staircase in the castle courtyard. In the thirteenth century military architects further provided against surprises by defensive outworks. The gate of Laon, at Coucy, so admirably described by Viollet-le-Duc, is a famous example. These outworks, which were called barbicans, were designed to protect the great gate and its approaches. [Illustration: 187. CARCASSONNE. GATEWAY OF THE LISTS, KNOWN AS THE _PORTE DE L'AUDE_] Around the walls of the city of Carcassonne a second line of ramparts had been drawn by St. Louis, in which only a single opening gave access to the _lists_ (Fig. 187)--that is to say, the space between the inner and outer enclosures. He afterwards built a huge tower, known as the _Barbican_, to the west of the castle, with which it was connected by crenellated walls, and by inner cross-walls, so arranged in a kind of echelon that the open spaces on one side were masked by the projections on the other (see plan, Fig. 167). The tower was destined to cover sorties from the garrison, and to keep open communication by the bridge across the Aude. It was rather an outwork than a barbican such as Philip the Bold built before the _Porte Narbonaise_, on the east of the city, towards the close of the thirteenth century. [Illustration: 188. CITY OF CARCASSONNE. GATE KNOWN AS THE _PORTE NARBONAISE_] The _Porte Narbonaise_ bears a general resemblance to the main gate of the castle, subject, however, to the great advance made in military architecture in the course of a century. The gateway towers are provided with spurs, an invention directed against the attack of miners, which had the further advantage of interfering with the action of a battering-ram, by exposing those who worked it to missiles from the adjacent parts of the curtain. The gate opened immediately upon the lists; it was defended by the crenellated semi-circular barbican, which was united on either side to the embattled parapet of the lists. Access to the barbican was obtained only by a narrow passage preceded by a bridge, the latter easily defended by a redan which adjoined the postern of the barbican. The gate itself was provided with two portcullises like those of the castle gate; behind the first were massive folding-doors, and over it a wide machicolation.