We were pretty thrilled that we arrived in the area in the late afternoon with just enough time to visit Shaubak Castle the evening before we went to Petra.  This made it our first stop in Jordan, and it was pretty cool to see the ruins of a crusader castle and to ponder the history there.  Its hilltop location is great with an interesting overview of the desert landscape. All that said, there isn't really much to see in the way of castle remains here.  One main tower and Shaubak's famous stairway/passageway are pretty much what there is to see.  We later visited the Karak (Kerak, Al Karak,...) crusader castle, and I much preferred that one -- there was just a lot more to see and explore there. (There's also a lot more to see at other crusader castles such as Belvoir in Israel.)