The Roman Empire (509 BC – 27 BC) was set at the coast of Tiber River and had a complete program of urban set pieces, structured on arches and vaults and articulated with marble columns. Rome hosted a wide range of people thanks to their magnificent temples, colonnaded streets, markets, theaters, aqueducts, baths, triumphal arches, and colonnaded plazas. After roman generals conquered lands, they started to impose architectural projects to those conquered lands to show their power of empire. They created grand colonnaded enclosures by borrowing models from Hellenistic cities and they created a new architectural range of soaring vaulted interiors. During Rome’s first two centuries, they inspired of the architecture of the more developed Etruscan culture. Etruscan’s architects brought with them the techniques of arches and vaults. Etruscans had arched gates at passages as a symbol, as a building technique.
Roman conquest began with its absorption of the neighboring Etruscans and Greek settlements in Italy. During the 3rd and 2nd centuries BCE, Rome was so powerful and wealthy that Julius Caesar and his adopted heir Augustus, dismantled the republican system, and Augustus became the first Roman emperor. Then, Augustus, used this imperial patronage to transform the city. Through the founded hundreds of cities under both the republic and the emperors, the Romans learned orthogonal urban design from Greek colonies, and from the Etruscans who built towns with the Greek right-angled surveying techniques. Some of the Roman cities had colonnaded streets on a strict grid and followed a grid.
Pompeii was acquired as a colony of Rome in 80 BCE. Pompeii contained the best preserved examples of Roman architecture. Roman civic centers like Pompeii always included a forum, a temple, and a basilica. The Forum of Pompeii, as the principal public space, showed the Roman preference for axial orientation. Unlike the casual openness of the Agora in Athens, the Roman’s political space Forum had large colonnades which enclose the third of the area tightly. The Temple at the north end of the Forum of Pompeii was standing on a high podium so it accessed by a single flight of stairs that enforce the axiality of the forum. Additionally, unlike Greek temples, Roman temples and arcades had engaged columns not for structural but for decoration. The Basilica of Pompeii used for legal and adminstrative business and appeared at the other end of the forum, in the southwest corner. Also, Aqueduct was a waterway as a bridge and the representation of how the arch became construct itself.
Beside, the Romans resembled other ancient people in their need to build temples and tombs, they also produced theaters and baths under the concepts of leisure society, pleasure, socialization, and hygiene. Unlike the Greeks, who inserted their theaters into the contours of sloping sites, the Romans built freestanding monuments using arches and concrete vaults. Moreover, the Romans invented a new form of theater called the amphitheater, which was generated in ellipse form by joining two theaters end to end. The Colosseum (an amphitheater in Rome), was Rome’s greatest place of spectacle and the empire’s largest building.
When we look at the Roman’s domestic architecture, we see the Roman house and call it as domus if it is in the city. If it is a country side house and not in the city then it is called as villa where wealthy Romans live. Additionally, while Roman colonial cities adhere to an order, the Rome developed without a plan. The colonnaded streets, the major avenues which defined by colonnades covering the side walks with shops on sides reflects the Roman architecture, however, Rome city did not have them.
The urban renewal of Rome began as a means of spreading the wealth acquired through conquest. Explicit use of architectural patronage began with a general: L. Cornelius Sulla, continued with the emperor Domitian and several projects were produced. Imperial Rome turned to a city of marble from a city of brick, plus, in Rome, imperial architecture went beyond exterior form to become the art of interior space.
I suggest you to watch the video below. It has a video simulation of ancient Rome and it explains the city regarding architecture.
Reference: World Architecture: A Cross-Cultural History by Richar Ingersoll.