The Cerro de los Ídolos ("Hill of the Idols"), dedicated to sun worship, was also used as an astronomical observatory. It is reached on a pleasant wooded trail leading to a platform with a magical view over the valley. This Aztec sanctuary contains a remarkable troglodyte temple, the Cuauhcalli, as well as several buildings, including the Templo del Sol, where one can observe the sun on two days every year (12 February and 29 October) as it rises through a gap in the mountains.