This church, in Parque Martí in the center of Guanabacoa, is also known as the Iglesia de Nuestra Señora de la Asunción. It was designed by Lorenzo Camacho and built between 1721 and 1748 with a Moorish-influenced wooden ceiling that's still beautifully intact.
The gilded main altar and nine lateral altars are worth a look, and there's a painting of the Assumption of the Virgin at the back. In typical Cuban fashion, the main doors are usually locked; knock at the parochial office out back if you're keen to see inside.