Limes

This picture shows part of the fortifications around the Roman Castel Saalburg

In his extension  the Limes is an impressive testimony of the Roman Empire. The Limes was protecting the border, stretching from the Rhine near Koblenz, about 550 miles through the present-day southwest Germany to Regensburg. The systematic expansion of the Limes began around 100 AD - several decades after a Roman army was defeated by Varus  -

The Limes, however, was not designed as an untouchable and insuperable line. Most of the border was a palisade of oak piles, and later in some places there was a ditch and an earthen wall about two meters high added.
Besides serving as a military "early warning system" the Limes served  as a "customs border" and as a "market places" for foreign trade.

The limes had more than 900 towers. The downturn of the Limes began 233 AD, when a number of legions were withdrawn for the war against the Persians. The Germans took advantage of the weakness and immediately started regularly plundering the Roman provinces. Around 260 AD, the border was finally overrun, so that the Romans finally gave up the disputed area.

Prosperous provinces within a few decades now became impoverished areas where Germans settled gradually. For the area between Limes, Rhine and Danube, from the year 297 AD  the term "Alemannia" appeared.