The Pantheon is considered to be a free-standing building. However, at one time there was another building on the back side, which abutted it and helped to buttress the rotunda. Records indicate that there was no interior passage between the buildings. It is estimated that it took over 700 workers to construct the Pantheon over three years. The original purpose of its construction are unknown, however, some indication was that the Pantheon was designed as a temple to all Greek gods.
In the original construction, the sides and the back of the Pantheon were flanked by other buildings. On the south side, it is speculated that a building known as the Basilica of Neptune was constructed at the same time as the Pantheon. On the east side, the walls of Saepta Julia (ostensibly a place for citizens to cast votes, but later used by Augustus to stage gladiatorial fights) spread to the outer face of the rotunda. Some of the original wall is still visible next to the Pantheon.
Documents indicate that the entire building was faced with marble, so little if any of these bricks would have been visible.
These are some of the remnants of the Basilica of Neptune. The brick support arches were to relieve some of the stress from the massive weight of the dome.
Possible foundation remnants of the Basilica of Neptune that was constructed next to the Pantheon.
Since the original plan for the Pantheon may have been as a temple to the Greek gods, it is not surprising to find remnants of pedestals on which may have stood statues to these gods.