While not built for particularly pleasant reasons, the Colosseum is still an amazing work of art. It was built c. 72 AD and was called the Amphitheatrum Flavium. We went inside and walked around. It's absolutely enormous and even with all the tourists and tours going through it didn't feel crowded. The wood that was the floor of the area has, naturally, rotted away, leaving the underneath where they housed animals and such exposed. Quite an amazing network of rooms and passageways.