The expansion of the city way beyond the ancient Servian Walls and the fear of invasions by the German peoples that pressed on the borders of the Empire induced the Emperor Aurelianus to build a new boundary wall. The works, started in the year 271 AD and carried out rather rapidly, were completed by Probus (276-282 AD). The walls are made of bricks and have an overall length of about 19 kilometers and are provided with a square tower every thirty meters (one hundred roman feet). The height of the wall is of about 6 meters, the width of 3.5 meters. The most important gates presented two gateways with arches decorated with travertine placed between two semicircular towers; other gates presented a simple arch, while the less important ones were simply inserted between the two square towers in the stretch of the walls. The haste in building the defensive system caused some large size buildings, such as the Praetorian Camp, the Castrensis Amphitheater, the Caestian Pyramid, the wall enclosing the Acilii Gardens ("Muro Torto") and some stretches of aqueducts, to be incorporated within the boundary wall.The walls were reinforced several times; the height was doubled in the years 401-402 AD under Honorius and Arcadius and the previous open-air path was replaced by covered corridors; above these a new open-air walkway furnished with merlons was created, where the war machines were kept. Some of the double gateways were also reduced to one only, and the towers were raised and reinforced, as in the Porta Appia (now called Porta S. Sebastiano) and the Porta Flaminia. Some of them became actual self-sufficient fortresses, with an internal storm door connected through two tenailles with the main walls, as can be seen in the only example still conserved at the Porta Ostiense, that has a storm door that dates back to the time of Maxentius. Other restorations of the walls were performed in the VI century by Belisarius, in the course of the Gothic wars.