This is a one to one scale replica of the ancient Roman colosseum. We just recently finished building the main structure and finishing the seating all the way around (holy crap that was a lot of drawing ovals). This build has taken us about 2 weeks so with anywhere from 2-4 people working on it at a time. We are really happy with how it turned out and have already had a handful of amazing arena battles.
In the middle of the arena floor is a mob elevator (water elevator from the caverns below) that carries mobs from underground cages (dungeons) up to the arena floor. The only thing we have left to do is hook up a redstone control system to release different mob types into the arena when we so choose. (in most of the screenshots the elevator is temporarily covered with sandstone)
The dispensers on the roof are out of sight most of the time yet when the switch is thrown from the control area, they rain arrows down into the arena floor livening up the battle.

Additional Notes

To get to the Colosseum head EAST from spawn along the eastern bridge/road. Its not very far at all. Or you can take the stargate.