Of course, the 1538 house is not the oldest thing in town. The Château de Rohan in all it's present day splendor, has it's beginning as a 14c fort, with four round towers, two of which form the medieval side of the château. A third tower is on site but the walls were dismantled during the French Revolution. The fourth tower no longer exists, but a 100 year old massive juniper (?) tree takes it's place and is every bit as large as the tower would have been.
BTW, during the Revolution the freestanding tower was used as a prison, housing 250 prisoners.
Forgive any facts here that might not be totally accurate. Do the research if you are interested.