This is a donjon or keep, part of a complex called the Chastel Blanc. What we see now probably dates from reconstructive work after an earlier version, dating from around 1112, had been demolished by Nur al-Din during campaigns in 1167 and 1171. That work was executed about 1188, then an earthquake in 1202 necessitated further improvement, so roughly the current donjon is from the 13th century. It fell into Muslim hands in 1271 when Sultan Baybars took it, when on his way to Krak des Chevaliers.
The tower was at the heart of a complex of two series of walls. The tower housed the garrrison as well as a church. The lower floor contains the church (Crusader Church of St. Michael), with only five windows (firing slits) for light. A floor higher a room housed the garrison. From there one can reach the roof. In this series I include pictures taken from there. It seems on a clear day one can see the Krak des Chevaliers.