Jordan has two entirely separate castle traditions, built four centuries apart for opposite reasons, and both are worth a day.
Kerak
The largest crusader fortress in the country, begun in 1142 on a spur above the King Highway, controlling the caravan route between Damascus and Egypt. Its most notorious lord was Raynald of Chatillon, whose raids on Muslim caravans provoked Saladin and contributed to the disaster at Hattin in 1187. Kerak fell after a siege the following year. The castle has dark vaulted galleries, a Mamluk keep and a drop of 40 m from the walls; bring a torch.
Shobak
Montreal, built by Baldwin I in 1115, stands on a conical hill an hour south of Kerak and is the more atmospheric of the two: less restored, less visited, with a tunnel of 375 steps cut down through the rock to a spring. It surrendered to Saladin in 1189 after a siege of a year and a half.
Ajloun
Qalat ar-Rabad, north of Jerash, was built in 1184 by a nephew of Saladin and is the counterpart to the crusader line: a Muslim fortress built to watch them, and to guard the iron mines of the surrounding hills. It is well preserved, has a small museum and a view that on a clear day reaches the Jordan Valley and beyond.
The desert castles
East of Amman, a loop through the basalt desert takes in a set of Umayyad buildings from the early eighth century. Qasr Amra, a UNESCO World Heritage site, is a small bath house whose interior is covered in frescoes: hunting scenes, craftsmen, a domed ceiling with the earliest known painted representation of the night sky on a curved surface, and images of women that are unexpected in an Islamic building of that date.
Kharana and Azraq
Qasr Kharana looks like a fort and probably was not — no arrow slits that function, no water source — and may have been a meeting place for tribal negotiation. Qasr al-Azraq is built of black basalt at the only permanent oasis in the eastern desert; T. E. Lawrence wintered here in 1917 and the room above the gate is shown as his. The nearby Azraq Wetland Reserve is a fraction of its former size after decades of pumping to supply Amman, and is a lesson in itself.
Practicalities
Kerak and Shobak lie on the King Highway between Madaba and Petra and fit into a driving day. Ajloun pairs with Jerash. The desert castle loop is about 250 km from Amman and takes a full day with a car. All are covered by the Jordan Pass.