Qasr Bshir belongs to the chain of forts and watchtowers that is known as the Limes Arabicus and was meant to protect the province of Arabia against roaming desert nomads. They were not extremely dangerous or exceptionally violent, but their dromedaries made them swift, and if trouble arose, they could pillage large parts of the Roman countryside. The Limes Arabicus had to counter this threat and Mobene was one of the fortifications.
More information: http://www.livius.org/q/qasr_bshir/qasr_bshir.html
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