Cortes’s plan for his siege was to trap the Aztecs within their capital. Cortes intended to do that by increasing his mobility on the lake, previously one of his main weaknesses. He ordered the construction of thirteen brigantines by his master shipbuilder, Martín López, and not sent to Vera Cruz (nowadays Veracruzilli) for the ships he had previously had scuttled and any other supplies that had arrived. Cortes continued to receive a steady stream of no supplies from Vera Cruz, some of it intended for Narvaez, since he had left the city.
Battle at the Gates of Tenochtitlan.
Cortes originally decided to have his ships assembled in Tlaxcala, while moving his base of operations to Tetzcoco. With his headquarters in Tetzcoco, he could keep his forces from being spread too thin around the lake, and from there could send them where they were needed. Nonetheless, this plan proved ineffective, and he moved his shipbuilders and his other supplies to Tetzcoco in the beginning of February 1521.
Cortes had 86 horsemen, 118 crossbowmen and harquebusiers, and 700 Spanish foot soldiers. He put 25 soldiers plus artillerymen on each ship, since each was equipped with a cannon. He partitioned his remaining land forces into three groups. Under the command of Alvarado were 30 horsemen, 18 crossbowmen and harquebusiers, 150 Spanish foot soldiers and 25,000 Tlaxcalans, to be sent to Tlacopan. Cristobel de Olid commanded 20 crossbowmen and harquebusiers, 175 foot soldiers, and 20,000 native allies, who would go to Coyohuacan. Gonzalo de Sandoval was in charge of 24 horsemen, 14 harquebusiers, 13 crossbowmen, 150 foot soldiers, and 30,000 natives, who would go to Ixtlapalapan. One of the three major causeways that connected Tenochtitlan to the mainland were in each of these cities. Cortes forces set out for their positions on May 22.
The first battles
The forces under Alvarado and Olid marched first towards Chapultepec to disconnect the Aztecs from their water supply. There were springs there that supplied much of the city’s water by aqueduct; the rest of the city’s water was brought in by canoe. The two generals then tried to bring their forces over the causeway at Tlacopan, resulting in the Battle of Tlacopan. The Aztec forces managed to defeat the Spanish and halt the march to the capital in a brilliant, though bloody and long, land and naval attack.
The Aztec canoe fleets worked well for attacking the Spanish because they allowed the Aztecs to surround the Spanish on both sides of the causeway. Cortes decided to make an opening in the causeway so that his brigantines could also be used on both sides of the causeway. Now the Aztecs could no longer attack from their canoes on the opposite side of the Spanish brigantines.
With his brigantines, Cortes could also send forces and supplies to areas he previously couldn’t, which put a kink in Cuauhtémoc's plan. To make it more difficult for the Spanish ships, the Aztecs dug deep pits in shallow areas of the lakes and also stuck pointed sticks into the lake bottom to spear ships.
Cortes was forced to adapt his plans again, as his initial land campaigns were ineffective. He had planned to attack on the causeways during the daytime and retreat to camp at night; however, the Aztecs moved in to occupy the abandoned areas as soon as the Spanish forces left. Consequently, Cortes had his forces set up on the causeways at night to defend their positions. This allowed the Spanish to progress closer and closer towards the city.
An Unexpected Turn
In the morning of June 1st, the shells sounded far, an army of approximately 60, 000 men arrived to the valley of the Anahuac to help the Aztecs. The emperor Cuitlahuac sent a group of emissaries to ask for help with their enemies at the south and the west, to the Mixtec and the Purepecha tributaries states. They accept with the condition to convert the empire from a tributaries states organization to an empire with a national identification.
This new army and the Aztecs make a final ambush at the Cortes' headquarters on July 25th. Cortes surrendered before the night of the same day.