The castle of Vitré was originally built in the XI century and reconstructed in the XIX century. Like many other castles it looks better from outside. The inner yard is freely accesible.
Beautiful architectural composition at 34 Rue Embas. The tower on the right is the remains of the medieval walls.
And of course the old town with half-timbered houses of 15-17 centures. Brittany has three main "half-timbered" cities: Vannes, Rennes and Vitre. And the latter wins.
Getting there: by train (http://www.sncf.com/). If you have a car, then find a large parking lot near the train station.