Most important among the structures developed by the Romans themselves were basilicas, baths, amphitheaters, and triumphal arches. Unlike their Greek prototypes, Roman theaters were freestanding structures. The auditorium was semicircular, with movable seats at the orchestra level. Distinctly Roman innovation were the uniting of stage and auditorium as a single structure and the rich architectural embellishment of the stage itself. For the oval amphitheaters such as the colosseum, there are no known Greek precedents. The monumental or triumphal arch was also a purely Roman invention. The basilica, probably a Roman development based on the Greek temple, provided a large and relatively open interior space. From its original use as a Roman law court, the basilica form was adapted by the Christians for their churches.
The baths, while probably derived from Greek gymnasia, were constructed on a totally unprecedented scale, the complexity of their plan competing with the luxury of their detail. In the typical Roman dwelling, the rooms were grouped about the atrium, which, by means of an opening in its roof, also served as a court. Multistory houses in the larger cities, called insulae, anticipated modern apartment buildings, as can be seen for example at Ostia (3d cent. A.D.). A third type of Roman dwelling was the luxurious country villa built by wealthy citizens to escape the congestion and squalor of the cities.