The Aztecs were impressed by the enormous pyramid-shaped temples that the Mayans had built hundreds of years before, so they built similar stone temples to worship their own gods. The stone altar at the very top was where they carried out sacrifices. Temples were regularly added to. They built over the top of the existing temple, like a new shell, so they grew bigger over time.
The Great Temple in Tenochtitlan was enormous. It had two staircases leading to twin temples at the top, which were dedicated to two gods. It was rebuilt six times over the Aztec Empire’s 200 year history. The temple was destroyed by the invading Spanish in 1521, but ruins can be visited today in Mexico City.