After six hundred years of struggle for expansion under the Republic and the early Empire, the Romans undertook a huge program of border fortifications, called limites (singular: limes), to defend their possessions. The first limes was established in Germany, along the rivers Rhine and Danube, under the reign of the emperor Domit-ian (81-96). The program was continued by Trajan (97-117), Hadrian (117-138) and subsequent emperors.
The limes was a line of defense composed of fortified towns, forts (castra), camps (castella) and watch towers (burgi), linked together with ditches and earth walls (vallum) crowned with wooden stockades. Border towns, forts and camps communicated with each other by means of military roads.
Inherited from the civilizations of Greece and the ancient Near East, permanent Roman fortification already displayed all the main characteristics of future medieval fortification. The Roman town was enclosed by a wall, called a curtain. The wall was constructed of masonry about 2 to 6 m thick and was usually about 10 m high. The upper part of the wall was arranged as a wall-walk and protected by a breastwork as high as a man; the breastwork was pierced with openings (crenels), which allowed defending forces to hurl their missiles at the enemy without, and solid standing parts (merlons or pinnae), which provided shelter. Curtains were reinforced by towers, either round or semicylindrical, higher than the curtains and jutting out. Their roofs were either covered by timber and tiles or left as open terraces where artillery machines could be placed. The various stories of the tower were arranged as storerooms or dwelling-places. Windows in the tower let the light come in and the spears or missiles out.
Town-gates were not numerous. In a city with a regular ground plan, gates were placed in the lengthening of the cardo (main north-south street) and the decumanus (main east-west street). The gatehouse was a compromise between a military stronghold and a triumphal arch, with decoration, windows, galleries, and statues. It included two defensive towers, passageways for carts, and smaller doors for pedestrians; the passages could be closed by heavy doors and a portcullis (a strong wooden barrier which could be raised and lowered by machinery).
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