The Basilica of San Isidoro is a gem of Spanish Romanesque architecture. Built in the 11th century, it houses the Royal Pantheon of the Kings of León, known as the 'Sistine Chapel of Romanesque art'. The frescoes, sculptures, and religious treasures make it a must-see for history and art enthusiasts. The basilica is also an important pilgrimage site, dedicated to San Isidoro, the patron saint of León.