The Casa Montejo, occupying the southern face of the Plaza Mayor, was built in 1549 by Francisco de Montejo el Mozo, son of the founder of Mérida. It is the oldest surviving domestic colonial structure on the American mainland and is notable primarily for its extraordinary Plateresque façade — a dense relief carving in which armoured conquistadors stand on the heads of defeated Maya warriors, flanked by wild men and family crests. It is a confrontational piece of architecture that tells you exactly what the Spanish thought about their conquest.
The building is now a Banamex bank, but public access to the interior courtyard is free during banking hours and worth taking. The colonial interior has been carefully maintained. The ground floor retains much of its original character — thick stone walls, high ceilings, heavy wooden doors. There is a small museum area with information about the building's history and the Montejo family. The Plaza Mayor–facing façade is best studied in the morning, when the light falls on the stone carvings and you can see the detail without squinting.