From Cortés, Third Letter, 255–57

[When Cortés first saw Tenochtitlan, he recognized he could easily been trapped in the city if the Mexicas destroyed the bridges or causeways connecting it to the mainland. He soon built three brigantines, which the Mexicas destroyed before he could use them, necessitating the construction of the portable bridge he built after the Mexicas trapped the Spaniards in their quarters. When he got to Tlaxcala, he decided to build portable brigantines that could be carried overland in pieces and put together near Tenochtitlan. While he sent advanced parties to the Mexica capital, he left a captain in Tepeaca to assemble the brigantines carried overland by the Tlaxcalans. In April 1521, the captain received a letter from one of two Spaniards living with indigenous people supportive of the Spanish side.]

[Cortés, at the time, was surveying Tenochtitlan, planning to lay siege to it.] After having made the circuit of the lakes, and acquired much information that might be of use in investing the city of Tenochtitlan by land and water, I returned to Tezuco, where I provided myself to the best of my ability with men and arms, and hastened the completion of the brigantines, and formed a canal extending from near our quarters until it discharged into the lake. The distance from the place where the brigantines were put together and the head of the canal to the lake, is full half a league; and in this work fifty days were consumed, and more than 8000 persons employed each day, natives of the province of Aculuacan and Texcuco. The canal was about twelve feet in depth, and as many more in breadth, and was protected by a coating and a fence or paling throughout its whole length, so that the water that flowed in it was conveyed without loss to the lake. Thus the brigantines were able to be removed to the lake unattended by danger or labor; a grand work certainly, and worthy of admiration.

As soon as the brigantines were completed and launched into the canal on the twenty-eighth of April in that year, I reviewed our whole force, and found it to consist of eighty-six horse[men], one hundred and eighteen archers and musketeers, seven hundred and more foot[soldiers] with swords and buckler [shields], together with three heavy iron cannon, fifteen small copper field-pieces, and ten hundred weight of powder. Having finished the review, I charged and enjoined much on the Spaniards to observe and comply with the orders I should give them in conducting the war with as great strictness as possible; and that they should take fresh courage and spirits, since they saw that our lord was leading us to victory over our enemies; for they knew that when we entered Tezcuco, we had not brought more than forty horse[s], and that God had succored us beyond our expectations, ships having arrived with horses, men, and arms, as they had seen; and that they should consider especially, that we were fighting in behalf and for the spread of our faith, and to reduce to your Majesty's service the lands and provinces that had rebelled; a consideration which should inspire them with courage and zeal to conquer or die.