Materials

Tile covered concrete quickly supplanted marble as the primary building material and more daring buildings soon followed, with great pillars supporting broad arches and domes rather than dense lines of columns suspending flat architraves. The freedom of concrete also inspired the colonnade screen, a row of purely decorative columns in front of a load-bearing wall. In smaller-scale architecture, concrete's strength freed the floor plan from rectangular cells to a more free-flowing environment. Most of these developments are ably described by Vitruvius writing in the first century AD in his work De Architectura.

The interior of the Pantheon in the 18th century, showing the concrete dome

Although concrete had been used on a minor scale in Mesopotamia, Roman architects perfected it and used it in buildings where it could stand on its own and support a great deal of weight. The first use of concrete by the Romans was in the town of Cosa sometime after 273 BC. Ancient Roman concrete was a mixture of lime mortar, pozzolana, water, and stones, and stronger than previously-used concrete. The ancient builders placed these ingredients in wooden frames where it hardened and bonded to a facing of stones or (more frequently) bricks. When the framework was removed, the new wall was very strong with a rough surface of bricks or stones. This surface could be smoothed and faced with an attractive stucco or thin panels of marble or other coloured stones called revetment. Concrete construction proved to be more flexible and less costly than building solid stone buildings. The materials were readily available and not difficult to transport. The wooden frames could be used more than once, allowing builders to work quickly and efficiently.

Cave canem mosaics ('Beware of the Dog') were a popular motif for the threshold of Roman villas.

On return from campaigns in Greece, the general Sulla returned with what is probably the most well-known element of the early imperial period: the mosaic, a decoration of colourful chips of stone inset into cement. This tiling method took the empire by storm in the late first century and the second century and in the Roman home joined the well known mural in decorating floors, walls, and grottoes in geometric and pictorial designs.

Though most would consider concrete the Roman contribution most relevant to the modern world, the Empire's style of architecture, though no longer used with any great frequency, can still be seen throughout Europe and North America in the arches and domes of many governmental and religious build

The Arch and the Dome

Aqueduct of Segovia

Roman bridge at Chaves, Portugal

The Roman use of the arch and their improvements in the use of concrete and bricks facilitated the building of the many aqueducts throughout the empire, such as the magnificent Aqueduct of Segovia and the eleven aqueducts in Rome itself, such as Aqua Claudia and Anio Novus. The same idea produced numerous bridges, such as the still used bridge at Merida.

The dome permitted construction of vaulted ceilings and provided large covered public space such as the public baths and basilicas. The Romans based much of their architecture on the dome, such as Hadrian's Pantheon in the city of Rome, the Baths of Diocletian and the Baths of Caracalla.

Remains of the baths of Diocletian, Rome. Etching made by Etienne_Du_Pérac in the 16th century.

Art historians such as Gottfried Richter in the 20's identified the Roman architectural innovation as being the Triumphal Arch and it is poignant to see how this symbol of power on earth was transformed and utilised within the Christian basilicas when the Roman Empire of the West was on its last legs: The arch was set before the altar to symbolize the triumph of Christ and the after life. It is in their impressive aqueducts that we see the arch triumphant, especially in the many surviving examples, such as the Pont du Gard, the aqueduct at Segovia and the remains of the Aqueducts of Rome itself. Their survival is testimony to the durability of their materials and design.

Housing

Remains of the top floors of an insula near the and the CapitoliumAracoeli in Rome.

Although less visible level to the modern observer, ancient Roman developments in housing and public hygiene are impressive, especially given their day and age. Clear examples are public and private baths and latrines, and under-floor heating in the form of the hypocaust, double glazing (examples in Ostia Antica), and piped water(examples in Pompeii).

Possibly most impressive from an urban planning point of view are the multi-story apartment blocks called insulae that catered to a wide range of residential situations. These buildings, solely intended for large scale accommodation, could reach several floors in height. Insulates were often dangerous, unhealthy, and prone to fires. There are examples in cities like the Roman port town of Ostia, that date back to the reign of Trajan and show how Roman architects met residential needs in a variety of situations.

As an example, consider the housing on Via della Focette: a large-scale real estate development that catered to up-and-coming middle class entrepreneurs. Rather like modern semi-detached housing, these residences had repeated floor plans intended for easy, economical, and repetitive construction. Internal spaces were designed to be relatively low-cost, yet functional and with decorative elements reminiscent of the detached houses and villas to which the buyers might aspire later in their lives. Each apartment had its own terrace and private entrance. External walls were in "Opus Reticulatum" and interiors in "Opus Incertum", which would then be plastered and sometimes painted. Some existing examples show that a relatively popular choice of interior decor was to paint panels in alternating red and rainbow.

Public buildings

Roman arena at Arles, inside view.

Roman architecture was sometimes determined based upon the requirements of Roman religion. For example, the Pantheon was an amazing engineering feat created for religious purposes, and its design (the large dome and open spaces) was made to fit the requirements of the religious services. Some of the most impressive public buildings are the amphitheatres, over 220 being known and many of which are well preserved, such as that at Arles, as well as the progenitor, the Colosseum in Rome. They were used for gladiatorial contests, public displays, public meetings and bullfights, the last of which survives in Spain. They are among the most impressive remains of the Roman empire at its height, and many of them are still used for public displays and performances.

Lighthouses

Tower of Hercules

Many lighthouses were built around the Mediterranean and around the shores of their expanding empire, including the Tower of Hercules at A Coruña in northern Spain, a structure which still survives to this day. A smaller lighthouse at Dover, England also still exists as a ruin about half the height of the original. The light would have been provided by a fire at the top of the structure.

Architecture of ancient Rome

Political propaganda demanded that these buildings should be made to impress as well as perform a public function.^{[citation needed]} The Romans didn't feel restricted by Greek aestheticaxioms alone in order to achieve these objectives.^{[citation needed]} The Pantheon is a supreme example of this, particularly in the version rebuilt by Hadrian and which still stands in its celestial glory as a prototype of several other great buildings of Eastern architecture. The same emperor left his mark on the landscape of northern Britain when he built a wall to mark the limits of the empire, and after further conquests in Scotland, the Antonine wall was built to replace Hadrian's Wall.