At the apogee of the Roman Empire, its territory extended over more than five million square kilometers, between Europe, Asia, and Africa. Rome exercised power over a population of more than 70 million people, which equated to roughly 21% of the world population at the time. In fact, as we have already shown in another article, all roads led to the city of Rome. The great seat of the empire and the material and immaterial heritage left by it is immeasurable, and even today researchers seek to understand its full impact on the current world. From the beginning of its expansion in the 6th century BC until its fall in the year 476 AD, the legacy left by the Romans encompasses areas such as law, plastic arts, Latin (which originated many different languages), systems of government, and, importantly, architecture.
Architecture has the potential to symbolize power, wealth and greatness. The Roman Empire used its buildings to convey this notion through its temples, markets, government buildings, baths, bridges, and aqueducts. The remains of the buildings are a testament to the technology that dominated the period, as well as the power and resources of the Empire's glory days. Stones, wood, marble, and materials produced such as “Roman concrete,” bricks, and even glass allowed the buildings to stand. More specifically, the walls developed from materials like stone with dry joints and sun-dried bricks at the start of the civilization to more sophisticated walls, built with a core of concrete and baked bricks.
The oldest Roman walls were made up of rough stones of large and distinct dimensions, supported by one another, without using any type of mortar to join them. They are often called cyclopean, as it is said that they were so heavy that only cyclops (giants of Greek mythology) would be able to lift them. “For the more modest houses and buildings, the walls were built with stones or clay bricks dried in the sun. Adobe bricks are made by mixing earth (sand, silt and clay) with water and an organic material, such as straw or manure, and cut into small units so that it can dry quickly without cracking. Clay bricks dried in the sun were glued with mud.” 
With the development of stone cutting techniques, it became possible to build walls with blocks of similar and uniform sizes, arranged in rows. These walls were called Opus quadratum. This technique was used around the 6th century BC and, over time, the precision and accuracy of block cutting improved. Even after mastering other wall-building technologies, the Romans continued to use this technique, primarily because of its aesthetic appeal. The Romans used limestone blocks or volcanic tuff that were abundant in Rome and its surroundings.
Over time, the Romans discovered that by mixing stones, limestone, water and pozzolana (volcanic ash from the region around Naples), an extremely resistant material formed when dry. It is the ancestor of our concrete, known as cement or Roman concrete. The so-called Opus caementicium were between 60 and 90 centimeters wide and were built from wooden shapes filled with this mixture, creating a somewhat irregular appearance.
Due to the rusticity of the surfaces, other ways to coat the walls emerged, maintaining the interior as Roman concrete. The oldest form of this technique was the so-called Opus incertum, introduced around the end of the 3rd century BC, which used small pyramidal blocks that were placed outside the wall and which resulted in a surface that had no regular pattern, hence its name. Initially, it consisted of a more careful placement of the caementa (fragments of rock and small stones mixed with concrete), making the external surface as flat as possible. Later, the outer surface became even more flat, reducing the amount of concrete and choosing small, more regular stones.
Opus reticulatum, or reticulate pattern, is its successor, consisting of a network of small square blocks cut in the form of truncated pyramids with a square base. Often of very precise size, these were seated in diagonal lines ordered on thin layers of mortar, placed at a 45 degree angle. The use of this type of finish started at the end of the 2nd century BC and remained very common until the advent of opus latericium, a different form of masonry, which became more common.
The Romans developed brick making techniques that became the main building material in the 1st century AD for the walls of houses, Roman baths, and monuments. Opus latericium (Latin for "brickwork") is a form of construction in which bricks of thick structure are used to face a core of opus caementicium. The bricks had rectangular, triangular, and even round shapes, with each part of the empire building the pieces with different dimensions.
On the other hand, Opus mixtum consists of hybrid walls, usually with opus reticulatum at the angles and sides of opus latericium. “At the end of the Republic, the custom started to reinforce Opus Reticulatum with horizontal bands of bricks or flat tiles, also functioning as leveling rows and dividing the lattice into panels. Therefore, “mixed work” is the conventional name for opus caementicium coated with panels or strips of lattice and brick. This style was particularly widespread in the Flávio and Adriano periods.” 
Over time, variations of these walls are seen with other material pages or the inclusion of other materials, such as wood in walls like Opus craticium, which is a technique related to half-timbering, a timber framework with the wall infill of stones in mortar called opus incertum. Although this article is intended to give a very general overview of the main wall-building techniques in the Roman empire, studying and identifying these methods provides several clues to the history of ancient Rome and the different stages of Roman economy and society.