For some days a large party of men had been at work constructing movable towers, similar to those used, centuries before, in sieges. They moved on rollers, and were to be dragged by the Tlascalan allies. From their summits the soldiers could shoot down upon the housetops, from which they had been hitherto so annoyed. The towers were also provided with bridges, which could be let down on to the roofs, and so enable the soldiers to meet their opponents hand to hand. When the structures were completed, the Spaniards again took the offensive. The gates were opened, and the three towers, dragged by the Tlascalans, moved out. The Mexicans, astonished at the sight of these machines, from whose summits a heavy fire of musketry were kept up, fell back for a time. The towers were moved up close to the terraces, and the soldiers, after partly clearing them by their fire, lowered the light bridges and, crossing, engaged in a hand-to-hand fight with the Mexicans, and drove them from their positions. But from the lofty houses of the nobles, the Mexicans still maintained their resistance. The towers were not high enough to overlook these and, as they came up, beams of wood and huge stones were cast down upon them; striking with such force that it soon became evident, to those within them, that the towers would not hold together. They were dragged on, however, until a canal crossed the road. The bridge had been removed, and both the cavalry and the towers were brought to a standstill. The latter were abandoned, and Cortez ordered his troops to make a road forward, by filling up the canal with stones and wood from the houses near. While engaged in this operation, they were exposed to an incessant fire from every point of advantage in the neighborhood, and from the opposite bank of the canal. The work was, however, completed; and the cavalry, crossing, drove the Mexicans headlong down the great street; until they came to another canal, where the same work had again to be performed. No less than seven canals crossed the street, and it took two days of constant fighting before the last of these was crossed, and the whole street in their hands. Just as the last canal had been captured, Cortez, who was ever at the head of his men, received news that the Mexicans desired to open a parley with him, and that some of their nobles had arrived at the palace for that purpose. Delighted at the news, he rode back with his officers. The Mexicans requested that the two priests who had been captured in the great temple should be released, and should be the bearers of his terms, and discuss the negotiations.