The Danube limes – The frontier of the empire was called limes by the Romans, in any part of the world, in deserts, along the rivers and seashores. However the world originally meant the road which led to the borders between the Romans and their enemies. In Pannonia and Noricum (today Hungary and Austria) this road was the Amber road and the valley of the river Sava. Romans reached the Danube at the end of them. The meaning of the word changed in parallel with the changing of the historical background. Romans occupied the vicinity of Carnuntum and Sirmium only in the 1st century AD, but after 50 years, in the reign of Claudius (41-54 AD) they came into power in the eastern part of Transdanubia and the frontier was put forward to the Danube. Also in the same time Noricum became the part of the emipre.
The evolved frontier existed nearly 400 years. After conciliation of the conquered people, the army was stationed on the Danube limes by the end of the 1st century AD. This organisation of the defence system worked constantly in the next 100 years. The forts and watch-towers are linked together by the limes road. The ground plans of the forts are uniform, their shape is quadrangle with two main roads and four gates. The house of commander, the barrack blocks, the stores and the bath were organised in a defined order inside the forts. The forts were built of earthen and wooden constructions, the watch-towers were built of wood. The towers of the forts looked inside the fort: their purpose was not to defend the fort but for observation only. The foreign policy of Rome was still expansive.
Although the rebuilding of gate towers in stone, projecting outwards had been started earlier, the significant changes date after the Marcomannic Wars (166-180 AD). All of the forts were rebuilt in stone with projecting semi-circular angle towers and in the reign of Commodus new watch-towers were raised along the Danube.
This linear system has changed in the time of the tetrarchy (293-305). In this time besides the army stationed along the limes mobilized troops were also set up. The number of the army on the limes was cut back. Perhaps the fort of Pilismarót was built by this time: instead of the regular order this fort was based on a hilltop, and its groundplan is not quadrangular but follows the shape of the hill.
The last building campaigns date to the time of the Constantine dynasty (306-350) and the reign of Valentinan (364-375). Both fort of Visegrád and the fort of Március 15. Square (Budapest) were built in the former period. Afterwards the army was so few in number that some forts were changed into a small tower or fortlet (e.g. Azaum, Sibrik Hill in Visegrád, Cirpi) but some new fortlets were also built (Leányfalu, Budakalász). The most important territory must be the Danube Bend, where (in today Esztergom and Pilismarót) so much watch-towers were raised that the distance between them was only 750 m on the average.
After the death of Valentinan the people of the Migration Period could not be arrested ever again. In 377 AD a united army of Goths, Alans and Huns invaded Pannonia. They had been settled down in Pannonia to defend the limes but in 433 AD the province was finally occupied by the Huns of Attila.