The Pantheon

The building's dome, more than 43 meters high is most impressive. It was the largest dome in the world until 1436 when the Florence Cathedral was constructed.
At the top of the dome is a large opening, the oculus, which was the only source of light.

The front portico has three rows of 8 columns, each one with a diameter of 1.5m. A huge bronze door gives access to the cylindrical building. Its diameter equals the interior height of 43,3m. {Source: aviewoncities.com}