The Collegiate Church is one of the most well-loved churches by the inhabitants of Madrid! If you cross Calle Toledo, with one of the gates of Plaza Mayor on your right, you're in the best spot to view the entire "Colegiata de San Isidro" structure.
Four Corinthian columns protrude from the center of the granite façade; if it weren't for the two square-shaped bell towers flanking the church, the building might look like a palace.
The group of statues above the main entrance depicts San Isidro, the patron saint of Madrid, and his wife Santa María de la Cabeza. The imperial coat of arms recalls Philip II's sister, Maria di Spagna, who financed the building's construction in the 1500s. When she was widowed, Maria gave all her property to the Society of Jesus. Moreover, a Jesuit was also the architect of the church, and used the Church of Jesus in Rome as the model for his project.
Some time later, the church was elevated to "collegiate" rank and was dedicated to the city's patron saint. The Collegiate Church of San Isidro was the provisional cathedral of Madrid while its main temple, the Almudena Cathredral, was under construction, which as I already mentioned took over a century. So it may be by force of habit that the inhabitants of Madrid continue to hold this Church in their hearts as the true cathedral of the city!
FUN FACT: During the Civil War in 1936, the Collegiate Church of San Isidro was almost completely destroyed. Fortunately, the remains of San Isidro were saved because they had been hidden inside a wall. When reconstruction began after the war, the saint's remains were discovered almost by accident. The churches of Madrid have definitely been through it all!