Here is a detailed scholarly lecture, "Contexts for the Ara Pacis", from 1997 by Minott Kerr, given at Reed College:
The Ara Pacis consists of two parts, a sacrificial altar proper, and the surrounding precinct walls. The almost square enclosure which defines an area of about 35 by 40 roman feet stood on a base and had openings on the east and west sides. The altar complex was entered by a flight of stairs leading to the western opening. The irregular topography of the original site sloped upward toward the east so the opening in the east side of the precinct wall was at the level of the adjacent via Flaminia. Within, the altar proper stood on a stepped base at the center with a second flight of steps at the west leading to the sacrificial table.
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