These ruins are not the fruit of a false romantic whim. Their presence in the Retiro Park results from the interest of the politician Antonio Cánovas del Castillo, who exerted his influence so that the ruins of the small church of San Pelayo or of San Isidoro, from the 11th century, which stood outside the walls of the city of Ávila, could be placed in this park for their future preservation. These ruins had been acquired by the State in 1884 to adorn the gardens of the National Archaeological Museum. Following the objection of the San Fernando Academy of Fine Arts, the State handed them over to the Madrid City Council in 1896.

What can be seen today are the remains of the semicircular apse and the side portal. The former is composed of back-to-back half columns of the giant order, which span the entire front with semicircular arches opening in the middle on columns with capitals bearing vegetal motifs. The openings are reduced to narrow embrasures resulting in a prevalence of solid wall, so characteristic of Romanesque architecture. The portal is also semicircular, with a moulded arch that rests on columns, except for the extrados and intrados, which do so on pillars. The capital is continuous and also features plant motifs as decoration.