The siege was now prosecuted with the most determined vigor. The approaches were made along the three causeways. The natives had broken down the bridges and reared a succession of formidable barricades, and as they were driven from one by the irresistible force of artillery, they retired, with firmness worthy of admiration, to the next, there to maintain their post to the last possible moment. The brigantines approached the sides of the causeways and opened a destructive fire upon the valiant defenders, where the Spaniards were exposed to no danger in return. Thus for nearly three months, by day and by night, on the land and on the water, the bloody strife was continued. Cortez was astonished at the obstinacy and efficiency of the resistance effected by the besieged. Gradually, however, the besiegers advanced, carefully filling up behind them the gaps in the causeway, that they might easily, if necessary, effect a retreat. They were taught the necessity of this precaution by a terrible repulse which they at one time encountered. Guatemozin, with a quick military eye, perceiving that the causeway occupied by one of the divisions of the Spaniards was impassable behind the Spaniards from trenches unfilled, and broken bridges, and the ruins of barricades, ordered the Mexican troops to retire, to lure the Spaniards forward. He then collected an enormous force, dispatching some in canoes along shallows which the brigantines could not approach, and then, at a signal from the great alarm drum on the summit of the temple, whose doleful tones could be heard for miles, the whole mass, with frantic rage, stimulated by hope, rushed upon the foe. The sudden assault, so impetuous, and sustained by such vast numbers, was quite successful. The Spaniards were driven back in confusion, horsemen and infantry crowding upon each other, till multitudes were forced, pell-mell, horses, and cannon, and men, into the chasms. Here the natives, in their light canoes, fell furiously upon them. More than twenty Spaniards were killed outright, and forty, mangled and bleeding, fell alive into the hands of the victors. There was no possible escape for the captives from their doom. They were to be sacrificed to the gods.