One of the largest and best-preserved Crusader castles in the Levant, built in the 1140s and later held by Saladin's forces after a famous siege. It sits on a plateau 900 metres above sea level with clear views toward the Dead Sea on a good day, and its underground vaulted halls and stables are extensive enough that you can spend a couple of hours just exploring the tunnels alone.
Karak is about 2 hours south of Amman (140 km) on the King's Highway, roughly halfway between Amman and Petra — most multi-day King's Highway itineraries stop here on the way south.
Spring and autumn for comfortable walking, though the elevation keeps Karak noticeably cooler than the Dead Sea or Wadi Araba even in summer, so it's a reasonable midday stop on a hot King's Highway drive.
Bring a flashlight for the underground sections — several vaulted rooms and connecting tunnels have no lighting. The castle is built into the town itself, so it's easy to combine with a walk through Karak's old market streets rather than just seeing the fortress and leaving.