Buses from Amman leave from Trababour Bus Station to 'Ajloun whenever they are full. Your best bet is early in the morning, around 8 am as they become more scarce as the day progresses.
'Ajloun is not a very big town, the bus will drop you off in the center and in order to get to the castle, which is clearly visible from the town perched on a hill, you can either walk the 3 km or take a taxi there.
Ajlun Castle (also Rabadh Castle), an Islamic fortress, built during the period of the Crusades. The castle is located on the top of a mountain just outside the small city of Ajlun. The castle is an interesting maze of passages and levels, and offers a wonderful view of the surrounding area, northwestern Jordan, and off into Galilee.
The cost to access the castle is 1 JD.