The south-east corner of the medieval wall marks of the crusader’s castle and its fortifications. They built this defensive citadel after having conquered Byblos in 1104 AD and it dominated ancient city. The approach to the castle from the piazza is up a ramp supported by two small arches. On the left hand side of the ramp lie a series of groin vault rooms fronted by a terrace that were used by Maurice Dunand as his home while he excavated Byblos. Under this ensemble is a large barrel vault room which abuts the east medieval wall. The room is paved with large stones and covers a roman mosaic which continues north under the wall of the room and terminated in the remains of an apse in an empty lot, east of the Emir Yusuf Chehab mosque. The mosaics in the empty lot are visible from the outside.