Seven years after the reconquest in 1118, Alkal’a Nahar (Alcalá de Henares) to Muslims, thanks to the Toledo archbishop, Bernardo de Sedirac, King Alfonso VII donates this population, to the archbishopric of Toledo. During the Muslim domination, a fortress with an important suburb had been built on the nearby hills, known as Alcalá la Vieja which, from that moment, was gradually abandoned, to settle on the plain, next to the old Church of San Justo. The archbishops were aware of the historical and religious importance of this site and with their policies, they were reinforcing and attracting more population. Most of the researchers think that it was Ximénez de Rada (1209-1247), who began the construction of the headquarters of the Toledo archbishops in Alcalá de Henares, formed by a fortress and a walled enclosure, probably over what there was it has been an extensive agricultural farm. Associated with this, there are news of the existence of waterwheel, from the Muslim era in this place. On the other hand, there is a reference that places around 1300, the construction of a “viaje de agua” for water supply. Little else was known about the supply system of the Alcazar. Thanks to the research carried out, we have learned that the construction of said infrastructure is based on the ancient Arab technique, known as qanat. These are mines that drain the aquifer, about two kilometers upstream, to the north and transport the water to the walled enclosure. The main advantages of this system of supply, is that the water is captured far from the fortification and can also be tubed and taking advantage of the existing slope, maintaining the pressure, to make the water sprout above ground level, forming sources.