File:Inside The Colosseum - Flickr - Me in ME.jpg

This is a five image panorama from inside the Colosseum in Rome. Its design is quite similar or modern sports venues. The inclined areas are seating was arranged. It could hold about 50,000 people or more. Seating was according to status with the senators having the first tier, up to the top level where plebes were seated. Emperor Domitian added a tier that could be used, likely while standing, by slaves, women, poor people and so on. In this photo about 2/3 of the structure that was below the sand covered wooden floor is visible. More details below.

The arena itself was 83 meters by 48 meters (272 ft by 157 ft / 280 by 163 Roman feet)  It comprised a wooden floor covered by sand (the Latin word for sand is harena or arena), covering an elaborate underground structure called the hypogeum (literally meaning "underground"). The hypogeum was not part of the original construction but was ordered to be built by Emperor Domitian. Little now remains of the original arena floor, but the hypogeum is still clearly visible. It consisted of a two-level subterranean network of tunnels and cages beneath the arena where gladiators and animals were held before contests began. Eighty vertical shafts provided instant access to the arena for caged animals and scenery pieces concealed underneath; larger hinged platforms, called hegmata, provided access for elephants and the like. It was restructured on numerous occasions; at least twelve different phases of construction can be seen.] 

The hypogeum was connected by tunnels to a number of points outside the Colosseum. Animals and performers were brought through the tunnel from nearby stables, with the gladiators' barracks at the Ludus Magnus to the east also being connected by tunnels. Separate tunnels were provided for the Emperor and the Vestal Virgins to permit them to enter and exit the Colosseum without needing to pass through the crowds. Substantial quantities of machinery also existed in the hypogeum. Elevators and pulleys raised and lowered scenery and props, as well as lifting caged animals to the surface for release. There is evidence for the existence of major hydraulic mechanisms and according to ancient accounts, it was possible to flood the arena rapidly, presumably via a connection to a nearby aqueduct. However, the construction of the hypogeum at Domitian's behest put an end to the practise of flooding, and thus also to naval battles, early in the Colosseum's existence.