The Roman army had many different units with different functions. The legion was the Roman army's biggest unit that served as defense. A legion usually consisted of 4000 to 4800 combat ready troops. Each legion was divided into cohorts of about 600 soldiers each. The century was the basic battlefield unit, which consisted of 100 men. These soldiers were led by a non-commissioned officer called a centurion. There were many different ranks of centurions. Another unit was a maniple and it consisted of two centuries. Before the third century, Roman officers and soldiers sometimes rode horses to the place of battle and then dismounted to fight. However, the emperor Gallienus (253-258 A.D), introduced a fast, mobile striking force mounted on horseback. These were the cavalry, since they fought on horseback. The emperor Septimius Severus ( 305-307A.D) began to use auxiliary units that included units of mounted archers (sagitarii), lancers, and heavily armored cavalry (cataphractii).
The emperors Diocletian (284-305A.D) and Constantine (306- 337 A.D) officially divided the Roman army into the limitanei--troops that guarded the frontier--and the comitatensis--the mobile fighting force. The comitatensis was a highly mobile army that could be ready to go anywhere in the empire at a moment's notice.
Another part of the military became the Praetorian Guard, which was the imperial bodyguard force under the control of the Praetorian Perfect. These were assembled by the emperor Augustus in order to prevent an assassination such as that of Julius Ceasar. However, many Praetorian Perfects gained too much power and in fact assassinated several emperors themselves. The emperor Septimus Severus disbanded the Guard and assigned his own men for his protection. The emperor Constantine totally dismissed the Guard and put in the Scholae Palatinae or palace guard, which were organized and regulated much better than the old Guard. The Praetorian Guard then became a civil government. The Empire was divided into three large regions called prefectures, each one under the control of a Praetorian Perfect.
Another aspect of the Roman army was the castra or camp. This is where the legions stopped for the night or were stationed in for months or even years. The same type of scientific plan used to build Roman cities was used to build a castra. The two main streets, the Via Principia and the Via Praetoria led into the center of the camp into the legionary headquarters. All camps were constructed very similarly with the streets or buildings being in the same part of camp so that a soldier that knew his way around one camp would know his way around another camp even in pitch black darkness. Each soldier knew his job in building the castra and was expected to perform it immediately so that the castra could function as a small city.
The Romans used Greek and Phoenician designs in warships. They also added fighting towers and movable gangplanks for grappling with (and then boarding) enemy vessels. Roman warships had a sharp, reinforced beak or ram used for puncturing an enemy hull and a high curved stern at the back. In addition, long before modern military engineers developed the idea of using armored vehicles to protect soldiers, the Roman army developed armored divisions of their own. When attacking a heavily defended enemy position, the Roman soldiers would arrange their shields to form a box covering their heads and sides. This was called the testudo.