These arrangements being made, Suchet crossed the Ebro at Xerta by his pontoon bridge on December 15th with twelve battalions, and made a sweeping circular march to shut in Tortosa on the northern side, while five battalions more, under General Abbé, moved by the other bank to block the bridge-head, by which the city communicates with Valencia and the south. The operation was completed without resistance, save at the Col de Alba, where a post of 600 Catalans, placed to cover the road between Tortosa and the pass of Balaguer, was discovered. This small force was driven into Tortosa after a skirmish; two small convoys on their way from Tarragona by sea had to turn back, because the way into the city was closed. The siege of Tortosa was remarkable for its swift progress and complete success--it can only be compared with Wellington’s capture of Ciudad Rodrigo, just a year later, in this respect. The Army of Aragon arrived in front of the city on December 16th--the surrender took place on January 2nd. At first sight the problem set before Suchet appeared by no means likely to receive such a rapid solution, for Tortosa as a fortress had many strong points. The city lies on and around the end of a spur of the Sierra de Alba, which runs down to the bank of the Ebro. This spur at its termination is many-headed: three ravines divide it into four separate hills, of which the highest is that crowned by the castle. The other three die down into undulations, and leave between them and the Ebro a comparatively flat space, on which the lower part of the town is built. Its higher quarters lie on the slopes where the various hills begin to rise from the level. The old enceinte of Tortosa had consisted of a mediaeval wall running across the hills and down the ravines between them, so as to enclose nothing but the inhabited parts of the city. But after a famous siege during the War of the Spanish Succession (1708), a number of outer works had been constructed to crown the culminating summit of each hill, and keep future besiegers at a distance, while part of the inner ancient fortifications had been transformed into a series of bastions, so far as the ground allowed.