This tower formed part of the donjon of the fortress erected by Baldwin IV., count of Hainaut, about the year 1150.
The first line of fortification was the work of Francis I.; the second line and the donjon date back to the 11th century.
The exterior is diversified by arched recesses forming machicolations, and the same architectural feature is reproduced in the square tower which rises like a donjon above the building.
The single street of the island curves from the gateway up to the abbey, ending in flights of steps leading to the donjon or chatelet.
Of the old castle, called Nenagh Round, dating from the time of King John, there still exists the circular donjon or keep. There are no remains of the hospital founded in 1200 for Austin canons, nor of the Franciscan friary, founded in the reign of Henry III.
The fact was that between the kings inability to defend the kingdom, and the powerlessness of nobles and peasants to protect themselves from pillage, every man made it his business to seek new protectors, and the country, in spite of Charles the Balds efforts, began to be covered with strongholds, the peasant learning to live beneath the shelter of the donjon keeps.
In the north corner of the courtyard is the Salle des Etats, which, together with the donjon in the west corner, survives from the 13th century.
It suffered from being used as a donjon in the middle ages.
The nucleus of the stronghold is a donjon over 200 ft.
The chief buildings are the chateau, mainly of the 15th century, of which the massive donjon of the 11th century known as the Tour de Cesar is the oldest portion; and the abbey-church of Notre-Dame, a building in the Romanesque style of architecture, frequently restored.