The main core and support of the Roman army were the legions - formations made up exclusively of the Roman citizens. A legion included approximately 5 000 well-armed and trained infantrymen and 120 horsemen. Many craftsmen served in the army too: sappers, carpenters, joiners, brick-makers, stone-masons, potters, smithes, weapon-makers, but also the medicinmen etc. Therefore the total number of the soldiers reached the number up to 5500-6000. At that time soldiers were given an annual salary (soldier´s pay). The service in the army lasted 25 years, and after the soldiers could retire as veterans.

The legions marked by number and also a name, build their own permanent fortresses (castrum legionis). Those fortresses were constructed according the common principles and regulations. They were square or rectangular shape and the size of the area covered 20-25 hectares. The main axis of the forteress were the roads connecting the opposite gates and crossed exactly in the middle of the inner area. Here stood the most important buildings: seat of the officers (principia - 2) with a large hall (basilica), the sanctuary and the armoury, the building of a chief of the legion - legatee (praetorium - 1), and the buildings of the high-ranking officers (tribunus - 4-9). In their vicinity used to be the hospital (valetudinarium - 3), the workshops (fabricae - 10), the storeage rooms (horrea - 12) and the public baths (thermae). Barracks for the soldiers (centurae) covered the largest area, organised according to particular formations (cohors) in the regular street system. The defence system of the fortresses included trenches, ramparts and a wall with towers. The fortification walls and the other buildings inside the fortress were firstly built of wood and clay and later rebuild in stone.

Besides the legions, auxiliary formations were also an important part of the Roman army. Those were smaller cavalry (ala), infantry (cohors) or mixed troops including 500-1000 men, recruited from the residents of the provinces, who were, after a hard and 25 years long service, granted the Roman civic rights. These troops were named apart of numbers also by the name of the tribe or territory, from where the soldiers came, or by the weapons they used. Often they were named after the emperor who had founded the troop. The officers of these troops were Roman citizens appointed by the seat-holder of the province. The commanders of the cavalry troops (praefectus alae) – belonging to the elite of the auxiliary troops – were chosen from among  the Roman nobles by the Emperor himself.

The permanent forts of the auxiliary troops (castellum) were built according to the same plans and principles as the legionary fortresses. Howewer these were much smaller in size. The forts of the infantrymen (pedites) occupied a territory of 1,5 ha while the camps of the horsemen (equites) and mixed troops were because of the horse stables larger, sometimes even more than 3 ha.

The building of the North-Pannonian border

The armament and equipment of the Roman Soldiers

The role and importance of the Roman Army

Marcomannic wars on the Danube

The period of prosperity and decline

The last attempts and the decline of the Roman authority