@Robin4 You got it in one! It is Masada, as @Michelle53 also discovered.
Visiting it is incredible - a huge palace set in the midst of the most arid and inhospitable land I have ever visited. Lord only knows how many slaves died building it. The story of the siege and of the rebels who died there is incredibly stirring. Afterward we went for a swim in the Dead Sea, followed by a much cooler swim in a swimming pool.
My daughter and I were in Israel for a wedding. We stayed there a week, then went to Croatia for a week. It was an amazing trip, during which we swam in three seas - the Mediterranean, the Dead Sea, and the Adriatic. Memories from that trip will last a lifetime.