...columna M. Aurelii Antonini, which was also called columna cochlis and columna centenaria divi Marci, stood on the west side of the via Lata, opposite the campus Agrippae and was erected some time before 193 A.D. to commemorate the victories of Marcus Aurelius over the Dacians and Marcomani in 172-175 A.D.
It is a direct imitation of the column of Trajan, the height of shaft and capital being the same, 100 Roman feet. The shaft itself, 26.60 meters in height and 3.96 in diameter, is composed of twenty-six rings of Luna marble. It is hollow, and contains a spiral staircase of two hundred and three steps. The interior is lighted by fifty-six rectangular loopholes. The capital is of the Doric order, and was surmounted originally by statues of Marcus Aurelius and Faustina. These disappeared at some unknown date, and the present bronze status of St. Paul was erected by Sixtus V. The pedestal of the column is of great height, for a considerable part of it still rises above the present level of the ground, which in turn is 4.75 meters above the ancient travertine pavement.
The exterior of the column is adorned with reliefs, arranged on a spiral band which returns upon itself twenty-one times. These reliefs represent scenes in the campaigns of Aurelius and correspond to those on the column of Trajan, but are far inferior to them in execution, and have been much injured by fire and earthquake. The original reliefs of the pedestal were chiselled off by Sixtus V, who added the present marble decoration, some of which was taken from the Septizonium.
Vincenzo Fasolo, "The Campo Marzio of G. B. Piranesi".