Founded on the site of the 1531 battle that gave the Spanish the city, and later the first missionary college in the Americas — the Franciscans who established the California and Sierra Gorda missions trained here. Maximilian used it as his headquarters during the siege of 1867 and was held here before his execution. Guided visits take in the cloister, the water system fed by the aqueduct, and a tree in the garden whose thorns grow in the shape of crosses, which the guides will tell you has never been satisfactorily explained.