From the 1st century AD the Roman legions had military garrisons in the outlying provinces in order to protect the frontiers of the Roman Empire. More than half of the approx. 30 legions was stationed between the estuaries of the Rhine and the Danube along the northern frontier of the Empire between Rheinbrohl (near Bonn) and Kelheim on the Danube was considered to be exceptionally endangered. For that reason fortifications were constructed from 85 AD to protect this area - the so-called Limes. The older parts of the Limes consisted of a palisade with a ditch and ramparts. In other places a wall which was almost 3 m high and 1 m wide protected the border.
Along the 548 km-long Upper-Germanic-Raetian Limes there were many watchtowers. The older wooden towers were replaced by stone watchtowers in the middle of the 2nd century AD. The remains of their foundation can frequently still be seen and can be visited. These excavations and reliefs on the Trajan Column in Rome nowadays give one a reasonably original reconstruction of the stone forts. One example of a reconstructed Limes watchtower can be seen in the Saalburg near Frankfurt. The tower platform was reached by ladders inside the tower, and these could be quickly raised in the case of an attack.
In course of time settlements sprung up near the watchtowers. Many places names still remind one of their Roman origins. A similar fortified frontier like the Limes was built in Britain - Hadrianís Wall - where Emperor Hadrian (117-138 AD) cordoned off the north of the country.
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