Colosseum's were one of the earliest Roman designs/inventions. It was a gigantic arena with thousands of seats, kind of like a much bigger version of a sports stadium, except instead of seeing people play sports, you see Gladiators and supposed huge warrior beasts try to kill each other. Mythical creatures or not, they still had Gladiators and other fighters fighting to the death. They even had a sliding floor that would open up to reveal a complex battleground. They were and still are really advanced.