Famous for its beautiful form, Roman classical architecture was revolutionary in its use of engineering to take architecture to new heights. The architecture relied on two critical technologies: the arch and cement. The Roman's transformed the use of the arch, creating large freestanding structures. Reinforced with cement, Roman tenements towered several stories with shops on the ground floor and housing above. Roman circuses, or coliseums, seated over 100,000 spectators at their largest, and Roman aqueducts stood several hundred feet above ground at the highest.