These fortresses or pucaras, built on small hills overlooking the entire surrounding valley are found throughout the country and were used mostly as defensive military installations. This fortress called Rumicucho, near San Antonio de Pichincha and the Pululahua Crater was built by the Inca invaders in the 15th century as mostly a military outpost to serve as an offensive and defensive guard post in their struggle to subjugate the northern tribes of the Cayambe, Cochasqui and Caranquis Indians, who offered them bitter resistance. However apart from this most important military function, it also served ceremonial and astronomical purposes and it also known that a large settlement of tradesmen, workers and other people surrounded the installation at that time.