The Roman History · Amm. 23.5.2

It had formerly been small and insecure, till Diocletian surrounded it with lofty towers and walls when he was strengthening his inner frontier within the very territories of the barbarians, in order to prevent the Persians from overrunning Syria, as had happened a few years before to the great injury of the province.

Ammianus Marcellinus · translated by C. D. Yonge · 1911 reprint
C. D. Yonge translation (preface dated February 1862); G. Bell and Sons, 1911; Project Gutenberg #28587
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