Please note that Mommsen uses the AUC chronology (Ab Urbe Condita), i.e. from the founding of the City of Rome. You can use this reference table to have the B.C. dates
From: The History of Rome, by Theodor Mommsen
Translated with the sanction of the author by William Purdie Dickson
Caesar Defeated before Gergovia
Before entering however on this retreat, which was far from agreeable to his quick and confident temperament, he made yet a last attempt to free himself from his painful perplexity by a brilliant success. While the bulk of the garrison of Gergovia was occupied in intrenching the side on which the assault was expected, the Roman general watched his opportunity to surprise another access less conveniently situated but at the moment left bare. In reality the Roman storming columns scaled the camp-wall, and occupied the nearest quarters of the camp; but the whole garrison was already alarmed, and owing to the small distances Caesar found it not advisable to risk the second assault on the city-wall.
He gave the signal for retreat; but the foremost legions, carried away by the impetuosity of victory, heard not or did not wish to hear, and pushed forward without halting, up to the city-wall, some even into the city. But masses more and more dense threw themselves in front of the intruders; the foremost fell, the columns stopped; in vain centurions and legionaries fought with the most devoted and heroic courage; the assailants were chased with very considerable loss out of the town and down the hill, where the troops stationed by Caesar in the plain received them and prevented greater mischief. The expected capture of Gergovia had been converted into a defeat, and the considerable loss in killed and wounded-- there were counted 700 soldiers that had fallen, including 46 centurions--was the least part of the misfortune suffered.
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Reference address : https://www.ellopos.net/elpenor/rome/5-07-subjugation-west.asp?pg=121