What was Nimrod Fortress used for?

It was a castle built in the Middle Ages or the Crusaders era. It was built to guard a major access route to Damascus against armies coming from the west. Some pieces of literature say that the crusaders built the Fortress, while other literature and archaeologists claim that although the Crusaders held the castle for a while, they were not responsible for building or expanding it. These pieces of literatures credit Ismali'is for building it and Malik el-Aziz Uthman for expanding it.