stylistic:tone, quality:correction
Interior

Upon entry, visitors are greeted by an enormous rounded room covered by the dome. The oculus at the top of the dome was never covered, allowing rainfall through the ceiling and onto the floor. Because of this, the interior floor is equipped with drains and has been built with an incline of about 30 centimetres (12 in) to promote water runoff.

The interior of the dome was possibly intended to symbolize the arched vault of the heavens. The oculus at the dome's apex and the entry door are the only natural sources of light in the interior. Throughout the day, light from the oculus moves around this space in a reverse sundial effect: marking time with light rather than shadow. The oculus also offers cooling and ventilation; during storms, a drainage system below the floor handles rain falling through the oculus.

The dome features sunken panels (coffers), in five rings of 28. This evenly spaced layout was difficult to achieve and, it is presumed, had symbolic meaning, either numerical, geometric, or lunar. In antiquity, the coffers may have contained bronze rosettes symbolising the starry firmament.

Circles and squares form the unifying theme of the interior design. The checkerboard floor pattern contrasts with the concentric circles of square coffers in the dome. Each zone of the interior, from floor to ceiling, is subdivided according to a different scheme. As a result, the interior decorative zones do not line up. The overall effect is immediate viewer orientation according to the major axis of the building, even though the cylindrical space topped by a hemispherical dome is inherently ambiguous. This discordance has not always been appreciated, and the attic level was redone according to Neoclassical taste in the 18th century.