The Colosseum, or Amphiteatrum Flavium

 Seeing the Colosseum was not very high on my list ("I'll see it if everybody else wants to see it") and if I had not seen it, I would have lost the best chance probably that exists to appreciate the colossal (ahem) building prowess of the Romans.  The largest Roman amphitheater in the world was begun in 67AD, and the opening ceremonies lasted 100 days in which 5,000 animals were slaughtered.  The lengthwise axis measures 617 feet, or two football fields, and the outermost walls were 164 feet tall.  It could accommodate 70,000 spectators.  Besides the pyramids, it was the largest man-made construction in the world.  

The arena was used to satisfy the Roman's love of "Bread and Circus" until 523AD, and lay abandoned during the Middle ages, used primarily as a source for building material elsewhere, but in 1774, it was made into a memorial for Christian martyrs, a bronze cross was erected, preserving the arena from further ruin.

When Rome caught fire, Nero cast blame on the Christian sect that had arisen in Rome, and condemned many of them to death by herding them into the Colosseum and releasing lions, tigers, etc.  Nero's most infamous act was to dip Christians into tar, hang them on poles, set them on fire, and use them as human torches to light the way to his villa.  

So you are looking at some of the bloodiest stones extant.  

What a great entrance!  The Arch of Constantine is the largest and best preserved of all the 
Roman arches, and celebrates Constantine's military victory over his co-emperor, Maxentius in AD 312.
(Aren't you glad I looked all this stuff up?)

Temple of Venus and Roma
Dedicated to Roma, the tutelary goddess of Rome, and Venus, patron goddess of the Imperial house. 

A huge part of the wonder of the place is just getting in through the massive entrances. 

On a wall in an archway, a etched drawing of the the city of Jerusalem, 
as imagined in the 18th century. 

Showing the huge, tented ceiling which was drawn over the crowds

And the walk out to the open theater. 

It stuns the viewer (This was taken from a later viewpoint three stories up. 

There was an elevator for the disabled which our guide accessed, because I was walking so badly,
Thank God.  I couldn't have made it up those 2,000-year-old stone steps. 

Looking back towards the platform we first stood on, 
from three levels up.  
It was so rewarding to see it.  At least once.  
And that was probably enough.