What is the main reason that rome was built on hills

Answer: The hills provided a natural defense.

Rome was built on seven hills, all of which lie within the walls of the city. These are located east of the river Tiber, and they are the Aventine Hill, the Caelian Hill, the Capitoline Hill, the Esquiline Hill, the Palatine Hill, the Quirinal Hill, and the Viminal Hill. Other hills that are close to the city but not usually counted within the traditional group are the Vatican Hill, the Pincian Hill and the Janiculum Hill. The most likely reason for choosing this location for the city is that the hills provide a natural defense against invasions and attacks.