The ruin on the left of this photo is what remains of the chapel of Saint-Melaine. It was built in the 12th Century to replace an earlier chapel (circa 851), and "devastated" in 1562 by the protestants. (I'm sorry, but I can't give any more information on this devastation until my French reading skills are better).
The decorated column capitals were sold when the Chateau was restored at the beginning of the 20th Century and are now in the Cleveland Museum of Art. They can be seen here.