The Iglesia de la Merced is the most historically charged building in Rancagua. It dates from the late 18th century, making it the oldest surviving church in the city, and its role in the 1814 battle gave it the status of a national monument.
The whitewashed adobe façade and modest interior are typical of the late-colonial Chilean style. A small plaque marks O'Higgins's wartime use of the building. Set aside fifteen minutes — it is one block from the main plaza, an easy add-on to any walk.