I'm certainly not an architect or engineer but it seems to me the Colosseum has been built as a series of arches atop one another; at ninety degrees from one another, etc.  It is like a series of Lego building blocks used to create archways to construct this impressive building.  

You can see in this next image how a series of beautiful arches form each level of the structure which are stacked atop one another to create the building:

Here is a closeup of an arch from inside showing its construction from "flat" bricks and mortar:

Can you count the number of arches in this shot?  

On the way back to the ground level, I spotted  this  "internal" arch constructed of granite blocks and seemingly without using mortar.  The holes were said to be created by polished granite facing that was at one point fixed to the structure.  It was removed after it fell into disuse. 

More detail showing an arch on one of the "bulwarks" that was filled in  with more bricks:

Arches, arches, arches.... 

Back on the ground floor looking at some of the structure holding up the floor above:

An example of arches formed at 90 degrees from one another:

Looking from inside through an arch at a nearby structure, itself created from arches:

A parting shot of the Colosseum showing the different layers of the structure, and ... some arches:

That's it for the tour of the Colosseum.  I hope you enjoyed. 

Thanks for taking the time to look.  Did you remember to click on any image to see them in a larger format?  They look best that way!

Bye for now,

Adam