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
The Gauls were from the first disposed to settle in groups; there were open villages everywhere, and the Helvetic canton alone numbered in 696 four hundred of these, besides a multitude of single homesteads. But there were not wanting also walled towns, whose walls of alternate layers surprised the Romans both by their suitableness and by the elegant interweaving of timber and stones in their construction; while, it is true, even in the towns of the Allobroges the buildings were erected solely of wood.
Of such towns the Helvetii had twelve and the Suessiones an equal number; whereas at all events in the more northern districts, such as among the Nervii, while there were doubtless also towns, the population during war sought protection in the morasses and forests rather than behind their walls, and beyond the Thames the primitive defence of the wooden barricade altogether took the place of towns and was in war the only place of refuge for men and herds.
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Reference address : https://www.ellopos.net/elpenor/rome/5-07-subjugation-west.asp?pg=19