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 capital of Latium required another and more complete system of defence; they proceeded to construct the Servian wall. The new continuous city-wall began at the river below the Aventine, and included that hill, on which there have been brought to light recently (1855) at two different places, the one on the western slope towards the river, the other on the opposite eastern slope, colossal remains of those primitive fortifications--portions of wall as high as the walls of Alatri and Ferentino, built of large square hewn blocks of tufo in courses of unequal height--emerging as it were from the tomb to testify to the might of an epoch, whose buildings subsist imperishably in these walls of rock, and whose intellectual achievements will continue to exercise an influence more lasting even than these.
The ring-wall further embraced the Caelian and the whole space of the Esquiline, Viminal, and Quirinal, where a structure likewise but recently brought to light on a great scale (1862)--on the outside composed of blocks of peperino and protected by a moat in front, on the inside forming a huge earthen rampart sloped towards the city and imposing even at the present day--supplied the want of natural means of defence. From thence it ran to the Capitoline, the steep declivity of which towards the Campus Martius served as part of the city-wall, and it again abutted on the river above the island in the Tiber.
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Reference address : https://www.ellopos.net/elpenor/rome/1-07-hegemony-rome-latium.asp?pg=15