Karak is small enough to walk. The castle, old town, souk, and most restaurants are within a 20-minute stroll of one another along the ridge, though the slopes are steep — wear sensible shoes. Taxis are cheap if your hotel is on the modern outskirts.
For visiting the castle, an audio guide is available for hire from the booth at the entrance; several languages are covered. If you are driving on to Dana, Petra, or back to Amman, parking in town is straightforward except on Friday mornings when the souk and mosque fill the streets.