The desert castle Qasr Kharana in Jordan.
Its purpose remains unclear today. “Castle” is a misnomer as the building’s internal arrangement does not suggest a military use, and slits in its wall could not have been designed as balistraria, or arrow slits. It could have been a caravanserai, or resting place for traders, but lacks the water source such buildings usually had close by and is not on any major trade routes. ~ Wikipedia