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Each of the 76 archways was numbered to allow people to get in and out quickly and efficiently. The outer wall is estimated to have required over 100,000 cubic meters (131,000 cu yd) of travertine stone which were set without mortar held together by 300 tons of iron clamps Photographer: Date: 25/11/09 08:04 Resolution: 2848 x 2136 DSCF6981.jpg