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…showed last 75 words of 309 total…thick, and is hollowed out by a series of alternately rectangular curve niches or recesses. The only window in the building is a central oculus, 9.1 metres in diametre. Because the oculus is open to the sky, the floor is slightly concave with a drain at the center. The huge bronze doors (7 metres high) are the largest Roman doors to survive in place and remain in use. The portico is 33.5 metres in length and 18 metres deep.