Château-Gaillard is a handsome fifteenth-century mansion on the rue Noé in the heart of the old town, built for Duke Francis II of Brittany and one of the finest examples of late Gothic civil architecture in the region. It now houses the Museum of Natural History and Archaeology, with collections covering the prehistoric and Gallo-Roman periods in the Morbihan.
The building itself is as interesting as the collection — note the carved corbels on the facade and the elegant courtyard inside. The museum's prehistory section is particularly good, with finds from the megalithic sites around the gulf. Entry is inexpensive, and the building is worth a look even if you don't go in.