This is my favorite photo of the Colosseum. In sepia tones. I think what surprised me the most about ancient Rome is that it is built of brick and mortar. The Romans invented concrete, and it is a testament to their building prowess that much of it is still around. The Flavians built the Colosseum, Vespasian and his son Titus (who conquered Israel and used it's wealth to rebuild Rome after a brief civil war, more on him later). Here is a section of the inside of the Colosseum showing a high concrete to brick ratio.
And a nice cut away, almost cross-sectional view. From the upper level, you can spy on these guys.