The square tower is in all likelihood the oldest tower of the castle. This construction is typical of Roman dungeons of the twelfth century after the first building of castles on a clod of land or a rocky outcrop as is the case here. These rectangular or square buildings were characterized by very high and very thick walls (Fig. 31) . On the floors of the roof, there was also a wooden roof. The access of these defensive towers was generally done on the first floor. This is confirmed here by a staircase clearly visible on the Arenberg map (Fig. 30) . The first levels were vaulted in stone and the others separated by timber floors. These passive defense dungeons can be supplemented by active defenses, which is the case here with machicoulis observable along the roof on its representation in the map of Arenberg.

The 3D representation combines all these elements to reflect as best as possible this square tower, home to the Provost of the Dukes of Arenberg from the fifteenth century (Fig. 32) .

Fig.30 - Square tower - 1609 Source : Arenberg Map 1609	Fig.31 - The reference Roman dungeon - 12th century Source : "Initiation à l'architecture française" de Robert Bornecque, 2013. Presses Universitaires de Grenoble	Fig.32 - 3D Model Source : Blender 3D model of the author

Totally invisible today, only a few remains of walls remain in the cellars and the ground floor of the Institut Saint-Michel.

On the timeless projection (Fig. 33) below we can see the location of the square tower under the oldest buildings of the institute. There is also the Griffon Tower in the background. On the left we can see furtively the tower recently excavated in recent years by Christian Kellen and his team. The square in front of the institute was the lower court of the castle. ( Swing the mouse "in" and "out" on the next image to see the comparison )