Okay, this is not a castle ----nor are many of the Desert Castles of N E Jordan castles. Any old ruined site seems to be have termed 'castle'.
This place is probably a camel-caravan stop over which was rebuild by the umayyad in the eighth century. our guide suggested that there had been a roman fort there before and possibly older buildings before that.
The outer walls are impressive. Gun/arrow-loops are observation points ---to see what is happening outside rather than shooting at them.
The site, although ruinous, is fascinating with most of the lower storey walls intact. Upstairs is a different matter!
Take a torch and a good camera.
Limited toilets and not for those who find walking difficult.
Would I go there again? Yes ---with a good guide.
I visited Hammam Sarah and Qasr al-Hallabat as part of a tour (only transport) organised from Farah Hotel in Amman. We were three people in the car sharing the price and I paid 13 JD. The places we visited were Qasr Kharana, Qusayr Amra, Qasr al-Azraq and the Wetland Reserve, Hammam Sarah and Qasr al-Hallabat (in the mentioned order).
We had lunch in Azraq and the driver told us about a restaurant were tours could have lunch (and it was cheaper if we came from Farah Hotel, 3,5 JD), but we preferred to have a falafel at a caf?. They were very tasty and was 250 fils. We asked if it was possible to visit the Wetland Reserve (it was not in the original plan) and the driver was happy because then he could go to the mosque after letting us off at the Visitor Centre.