The square stone keep at the southern end of the citadel is the oldest standing structure in the complex and one of the earliest surviving rectangular keeps in France. It was begun about 1010 by Foulques Nerra of Anjou and completed by his son Geoffroy Martel. At 36 metres tall it dominates the southern half of the citadel.
You can climb to the top by way of the original spiral staircases inside the four-metre-thick walls. The interior is essentially unfurnished -- raw stone, narrow window slits, and the openings where wooden floors once divided the levels. The view from the wall walk takes in the whole of the citadel and the Indre valley to the south.