The Roman society spawned excellent civil engineers. They used their talents to create many civic improvements like roads, aqueducts and public buildings. Many of these achievements involved the use of arches. An arch is formed when the forces created by one stone curve cancel out the forces created by one or more adjacent structures. This concept allowed the Romans to use a series of arches to build the Colosseum, a freestanding oval structure best suited to the games, races, and physical contests that were the most popular types of entertainment in that period. The Colosseum was an architectural marvel of its day, not only because of its size, but also because of its clever high-tech features like stage elevators and trap doors that allowed performers to enter the space unexpectedly.