The walls of Palmyra are a series of protective fortifications that providing customs barrier as well as protection for Palmyra from invaders and Bedouins. The development process of the walls can be separated into three stages; the first dates to the first century AD, and the second dates to the reign of the Roman Emperor Aurelian in the third century. The third was an improvement of the Aurelian walls commissioned by emperor Diocletian in the fourth century.