Buses (or service taxis) from Amman leave from Tabarbour (Mahata Shamal) aka Northern Bus Station to Ajlun (Ajloun) whenever they are full. Your best bet is early in the morning, around 8 am, preferably not on a Friday when only service taxis (2.5-3JD - 2016) will be available. Buses become more infrequent as the day progresses. If you cannot find a direct bus to Ajloun, you can try to go to Jerash and from there take the bus to Ajloun. Renting a car from Amman for a day trip to Jerash and Ajloun from Amman will set you back JOD 60.
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 2-3 km or take a taxi there. If you plan to travel to Jerash afterward, consider hiring the taxi driver for the full day, but make sure you set a price in advance.
The castle was built in 1184-1885 AD and was one of the few fortresses built to protect the country against Crusader attacks from both the west and the north. It also controlled the area around it, including another of its major objectives, to control the iron mines in the area. The castle is one of the best preserved and most complete examples of medieval Arab-Islamic military architecture.
Ajloun Castle dominated the three main routes leading to the Jordan valley and protected the trade and commercial routes between Jordan and Syria. It became an important link in the defensive chain against the Crusaders who unsuccessfully spent decades trying to capture the castle and the nearby village.
From the top of the castle, visitors can enjoy panoramic views of the Jordan Valley and the highlands of north Jordan. Excavations have recently identified a church that was built on the castle site in the earlier Byzantine period, while restoration and conservation works have made all areas of the castle accessible and safe for visitors.
The cost to access the castle is 3 JD.
You can stay at the Ajloun Reserve. Prices start from JOD 70 for a double. The lodge is run by the national park. Accomodation can be booked at here.
There are also two hotels in Ajloun from JOD 25 for a double.