Where? The Colosseum Architecture

The Colosseum, although terrible acts went on inside it, was one of the most magnificent buildings of all time. It is completely free-standing, unlike most buildings of the tome, which were built into hillsides. It could fit anywhere from 50,000 to 87,000 people. It was divided into section, ranging from the Vestal Virgins and Emperor's boxes to the places for slaves, women, children and all common people. The arena itself was 83 by 48 meters, allowing enough room for elaborate gladiator shows involving animals, weapons, and chariots. Underneath of the actual arena is the hypogeum, a storing place for animals and slaves before the fights. Hydraulic technology allowed them to raise and lower props, animals, and other things onto or below the arena. The Colosseum is one of the best pieces of architecture of that time and now.

http://en.wikipedia.org/wiki/Colosseum#Physical_description