Lying between the confluence of the river Eresma and the river Clamores, the first documented record of the fortress dates back to Christian writings in the 12th century It was built on top of a rock, that is testament to its military status. It also served as a residence for Alfonso VIII. In the thirteenth century the building took on an air of Gothic elegance thanks to the work of the Spanish architects, John II and Henry IV. The last architectural phase was done in 1587 by the architect Francisco de Mora, possibly in collaboration with Juan de Herrera, who did the Main Courtyard and the Honour School. In 1764, Charles II founded the Royal College of Artillery here. The edifice has a number of secret passageways that lead to the river and connect several of the city's palaces together.