The correct answer to this open question is the following.
Unfortunately, you did not specify the letter Cortés sent to the king of Spain. Hernan Cortés wrote many letters. Which one you are referring to?
You also forgot to attach it so we can read it.
However, trying to help you, we can comment on the following.
The parts of Cortés's letter that would be most interesting to a cultural historian would be the ones where Hernan Cortes describes his first impressions about the Valley of México and his encounter for the first time to the great city of Tenochtitlan.
In this part of the letter, Hernan Cortes is astonished by what he was witnessing. A magnificent, clean, large, impressive city with clean water canals that connected to different parts of the city. Hardworking people farming their "chinampas," portions of land where they farm and live in those water canals.
He also described the magnificent downtown area with large and tall buildings, temples, and pyramids.
He let King Carlos V know the customs, culture, and traditions of the Aztec people, that although primitive Indians, they had advanced organization on trade and social systems.
Hernan Cortés arrived in the Americas on March 4, 1519. He specifically arrived at the Yucatan Peninsula, in the Mexican territory. Five months later, on November 8th, 1519, he finally arrived at the center of the Aztec Empire: the great city of Tenochtitlan, the capital of the Aztecs.
Cortés realized that Spaniards alone, it was going to be impossible to defeat the mighty Aztec Army. That is why in 1521, Cortés made alliances with Native Tribes that were enemies to the Aztecas. Specifically, the Chollulans and the Tlaxcalteans.
