The church of San Nicolás, like San Pedro El Viejo, very likely stands over the site of a prior mosque.
Moreover, its bell tower was probably once a minaret, as indicated by the horseshoe arches and the multilobed arches on it, two strong Islamic styles in Spain.
The tower is the oldest part of the church, dating to the 12th century and of Mudéjar construction, the style of Muslim craftsmen living under Christian rule. This would have been a common style in Madrid, from the 11th to the 15th centuries.
While this walk ends near the hilltop where 9th century Muslim Madrid began, there are also a few more treasures worth exploring a little further afield in medieval Madrid: the Museo Nacional de Arqueología and the ruins of San Isidoro’s chapel in the El Buen Retiro park.