Amphitheatres were outdoor arenas where people watched sporting events and plays involving slaves, prisoners, or animals. They were shaped with semicircular seating around a central space and built into hillsides with low back walls. Arches were developed in Ancient Greece and refined by Romans, allowing them to span spaces by distributing compressive stresses rather than tensile stresses. Romans widely used arches above ground in structures like aqueducts, which carried water through arched lines joined together to supply bathhouses and towns.