The Roman History · Amm. 24.2.12

But before it was quite dawn, the garrison perceived what was being done, with the addition that a violent stroke of a battering-ram had broken down a tower at one corner; so they abandoned the double city wall, and occupied a citadel close to the wall, erected on the level summit of a ragged hill, of which the centre, rising up to a great height in its round circle, resembled an Argive shield, except that in the north it was not quite round, but at that point it was protected by a precipice which ran sheer down into the Euphrates; the walls were built of baked bricks and bitumen, a combination which is well known to be the strongest of all materials.

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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