The images show two views of the Colosseum (Flavian Amphitheater). Roman innovations in concrete allowed the builders of the Colosseum to - create a multilevel freestanding structure with tiered seating that could be constructed quickly - include a massive velarium, or sunshade, that could be extended or retracted over spectators - construct a watertight rim around the pit for the purpose of flooding the arena for sea battles - organize seating according to the differing social status of the spectators