It was in the area of architecture that Roman art produced its greatest innovations. Because the Roman Empire extended over so great an area and included so many urbanized areas, Roman engineers developed methods for city building on a grand scale, including the use of concrete. Massive buildings like the Pantheon and the Colosseum could never have been constructed with previous materials and methods. Though concrete had been invented a thousand years earlier in the Near East, the Romans extended its use from fortifications to their most impressive buildings and monuments, capitalizing on the material’s strength and low cost. The concrete core was covered with a plaster, brick, stone, or marble veneer, and decorative polychrome and gold-gilded sculpture was often added to produce a dazzling effect of power and wealth.
Because of these methods, Roman architecture is legendary for the durability of its construction; with many buildings still standing, and some still in use, mostly buildings converted to churches during the Christian era. Many ruins, however, have been stripped of their marble veneer and are left with their concrete core exposed, thus appearing somewhat reduced in size and grandeur from their original appearance, such as with the Basilica of Constantine.
During the Republican era, Roman architecture combined Greek and Etruscan elements, and produced innovations such as the round temple and the curved arch. As Roman power grew in the early empire, the first emperors inaugurated wholesale leveling of slums to build grand palaces on the Palatine Hill and nearby areas, which required advances in engineering methods and large scale design. Roman buildings were then built in the commercial, political, and social grouping known as a forum, that of Julius Caesar being the first and several added later, with the Forum Romanum being the most famous. The greatest arena in the Roman world, the Colosseum, was completed around 80 A.D. at the far end of that forum. It held over 50,000 spectators, had retractable fabric coverings for shade, and could stage massive spectacles including huge gladiatorial contests and mock naval battles. This masterpiece of Roman architecture epitomizes Roman engineering efficiency and incorporates all three architectural orders—Doric, Ionic, and Corinthian. Less celebrated but just as important if not more so for most Roman citizens, was the five-story insula or city block, the Roman equivalent of an apartment building, which housed tens of thousands of Romans.
It was during the reign of Trajan (98-117 A.D.) and Hadrian (117-138 A.D.) that the Roman Empire reached its greatest extent and that Rome itself was at the peak of its artistic glory— achieved through massive building programs of monuments, meeting houses, gardens, aqueducts, baths, palaces, pavilions, sarcophagi, and temples. The Roman use of the arch, the use of concrete building methods, the use of the dome all permitted construction of vaulted ceilings and enabled the building of these public spaces and complexes, including the palaces, public baths and basilicas of the “Golden Age” of the empire. Outstanding examples of dome construction include the Pantheon, the Baths of Diocletian, and the Baths of Caracalla. The Pantheon (dedicated to all the planetary gods) is the best preserved temple of ancient times with an intact ceiling featuring an open “eye” in the center. The height of the ceiling exactly equals the interior diameter of the building, creating an enclosure that could contain giant sphere. These grand buildings later served as inspirational models for architects of the Italian Renaissance, such as Brunelleschi. By the age of Constantine (306-337 AD), the last great building programs in Rome took place, including the erection of the Arch of Constantine built near the Colosseum, which recycled some stone work from the forum nearby, to produce an eclectic mix of styles.
Roman aqueducts, also based on the arch, were commonplace in the empire and essential transporters of water to large urban areas. Their standing masonry remains are especially impressive, such as the Pont du Gard (featuring three tiers of arches) and the aqueduct of Segovia, serving as mute testimony to their quality of their design and construction.