In its prime, the Colosseum was a venue for gladiators to fight to the death, but today, the arena lives on as one of the grandest and most spectacular megastructures in the world. Constructed in the absence of electricity, power tools or earth-moving equipment, it reveals an architectural process that rivals today's modern methods. 2,000 years ago, its creators pioneered the use of building materials that are still with us today to build a structure that could hold more than 50,000 people.