It is hard to measure what this structure was like in comparison to modern arenas because so few seats remain. The last section can be seen just to the right of center in this picture. It doesn't help that a large section of the outer wall is missing.
Here is a higher view. The far end of the arena floor has been reconstructed.
This view shows the Palitine hill as seen through one of the passageways. Note the iron bracing at the left.
This arch on the inside of the outer wall had to be filled in so it would not colapse.
The Colosseum was declared a church by the Pope partly to remember the martyrs who died there and partly to keep people from removing the stone for other building projects. Note the crude carving of Christ in the keystone below the plaque.