Seaports were defended at the mouth by towers on either shore, between which chains, worked from within, could be stretched to bar the passage. The harbour of La Rochelle is thus protected. According to some archæologists of authority, the tower known as the _Tour de la Chaîne_ (to the left of the drawing) is older than that of St. Nicholas (on the right), which is supposed by them to have been built in the sixteenth century on the foundations of an earlier tower contemporary with that on the other side of the Channel. The piles upon which these towers stand seem to have given way in part, and to have caused a perceptible inclination of the Tower of St. Nicholas. [Illustration: 194. ENTRANCE TO THE PORT OF LA ROCHELLE. TOWER OF ST. NICHOLAS, AND TOWER CALLED _TOUR DE LA CHAÎNE_. BEFORE THE RESTORATION] The suggestion made in a very fanciful modern design, that the two towers were once united by a great arch, is wholly without foundation. Such a useless structure would have entailed defensive works equally useless, seeing that a chain stretched from tower to tower at high tide--at low tide the harbour was inaccessible--would have been perfectly effectual against any vessels of that period attempting to force a passage. _Bridges._--As is the case with all other architectural buildings, the origin of bridges dates back to the Romans, by whom they were often decorated with triumphal arches. The bridge of St. Chamas in Provence, known as the _Pont Flavien_ (Flavian Bridge), is an example which seems to date from the first centuries of the Christian era. The triumphal arches were in later times replaced by fortifications; they became _têtes de pont_, _bastilles_, or crenellated gate-houses, the function of which was not, like that of the arches, the decoration of the structure or the glorification of its founder, but the defence of the passage across the river, and the protection of the fortress with which it communicated. [Illustration: 195. BRIDGE AT AVIGNON. RUINS OF THE BRIDGE KNOWN AS THE PONT DE ST. BÉNÉZET] Among the bridges constructed by mediæval architects, that of St. Bénézet, the Bridge of Avignon, seems to be the most ancient. This bridge, which was begun about 1180, and completed some ten years later, is equally remarkable for its architectural details, and the structural problems solved by its builders. It crosses, or rather used to cross, the Rhone--for though the arm towards the _Rocher des Doms_ is the narrower, it is the deeper--on nineteen arches, extending from the foot of the Doms, on the Avignonese bank, to the Tower of Villeneuve, on the right bank, after a slight deflection southward. The gate-house on the left bank, some fragments of which still remain, is said to have been built by the Popes in the fourteenth century, for the purpose of levying tolls, a perquisite shared by them with the King of France.