The Romanesque brick Church of San Nicolás de los Servitas is the oldest in the city, built in the 12th century on the site of a former mosque. Much of it was rebuilt in the 15th through 18th centuries, but today inside we can still see the 15th-century apse, Gothic ceiling vault, and Mudéjar (Moorish-inspired medieval) plasterwork and other details as well as outside the original bell tower, adorned with Moorish-style brick horseshoe arches - though its slate spire was added in the 18th century. Inside there's also an exhibit about the early history of Muslim Mayrit. 

Read more in my post Discovering Madrid´s Little Known Medieval Legacy.