Desert Castles are old castles found around desert in Jordan.
Desert Castles are castles spanned in the desert, either completely standalone or with small towns around them. These castles once were getaway for Kaliffs from the Omayyad Period. There are too many of them, most ruined or off-limits to the visitor because they are under maintenance.
Most castles are accessible within a 1-day trip from Amman via the Desert Highway. Kerak is also accessible through the King's Highway. There's a small fee (around 1 - 2 JOD) for entrance in each of them, but your tour operator has likely reserved the tickets for you beforehand.
The only trip is the trip back to where you have been before visiting the castle, with the exception of Kerak which offers an exit from both the King's Highway or the Desert Highway.
The ruins for each castle. Noteworthy are:
Walk around and take pictures. What else would you do in a ruined castle?
Also go to the top of Al Kharaneh and watch the view (careful, it's dangerous).
Plenty of water. It's hot out there, especially in the summer.
From the very same road you got in.
As almost anywhere in Jordan, the chance that you are mugged or otherwise assaulted in the Desert Castles is very low. Use common sense about pickpockets.
Also watch your step, for some places in the ruins require stable shoes.