This fine old church is one of the few remaining windows on post-Muslim Madrid, most notably its clearly Mudéjar (a Moorish architectural style) brick bell tower, which dates from the 14th century. The church is generally closed to the public, but it’s arguably more impressive from the outside; the Renaissance doorway has stood since 1525. If you can peek inside, the nave dates from the 15th century, although the interior largely owes its appearance to 17th-century renovations.
The church is the focus of important Good Friday celebrations. Along with the Iglesia de San Nicolás de los Servitas, the Iglesia de San Pedro El Viejo is one of very few sites where traces of Mudéjar Madrid remain in situ. Otherwise, you need to visit Toledo, 70km south of Madrid, to visualise what Madrid once was like.