Architecture of the Roman Empire: The Pantheon was built as a temple to all gods, but over time it became a shrine to one single god - Jupiter. It was constructed in 125-124 BC by Marcus Agrippa, who also built many other impressive structures in Rome. The Pantheon has been called the most beautiful building in the world! It is a large, mostly open structure with an average diameter of 42 feet (13 m). There are actually two completely separate buildings inside the Pantheon - the outer shell or cupola was built first and it is where you will find the altar for sacrifices to Jupiter. The inner space is hollow and it is here that you will find the original stone roof which can be seen from outside the building. This ceiling still contains its original stucco decoration which includes images of planets, stars, and constellations. The Pantheon was originally built as a temple to all gods, but over time it became a shrine to one single god - Jupiter.