The Limes Germanicus (simply known as "Limes" in Germany) was an ancient great wall of the Roman empire along its border with Germanic tribes.
It's of special interest because it's a quite unusual defensive / border security system. This line of defence was in practice incapable (and later on likely also not meant) of defeating attacks. Even the possibility of successful counterattacks on raiders before they did harm was small.
The Romans hadn't enough troops to set up a solid enough defence capable of doing that. It wouldn't have helped if they were able because they also would have had to sustain the effort for generations and the border provinces were simply not worth the effort.
They had to settle with much lower ambitions, and they've set up a border security strategy that was much, much more complex and efficient than a simple "keep out!" defence.
The strategy included four major elements:
(1) mark the border to solidify the claim on land, control it
(2) trade and ally with nearby tribes as the "carrot" part of "carrot and stick"
(3) lower the expectation value for net raid profit with border security efforts
(4) use punitive expeditions as the "stick" in "carrot and stick"
This worked quite well until the resources spent on border security diminished (archeologists learned that the garrisons were only in partial use in the late imperial phase) and the Germanic pressure reached a new level (the barbarian migration pressure re-emerged about two decades after the establishment of the border).
The limes earthworks and "walls" weren't capable of stopping many raiding party infiltrations. Their greater value was in the difficulties such a wall and ditch posed for the exfiltration (return) of said raiding party.
The border guards were at alert by the time of exfiltration, the raiders were hunted by Roman cavalry and they were in a hurry (unable to choose the best time and place for overcoming the obstacle), laden with booty and possibly even occupied with captives (new slaves).
Only really strong raiding parties had a good chance to return with livestock. Infiltration and exfiltration on horse was also difficult.
This lowered the expectation value for net raid profits (3).
The Romans had early on an "all potential foes need to be destroyed" attitude. Such a limited ambition border defence as the Limes Germanicus turned out to be was therefore a great step back towards humility and practicality.
They had to secure their long Northern border with strictly limited resources and have apparently developed a smart and effective strategy for the challenge.
The Limes Germanicus was therefore an example of a linear defence (or security concept) with low ambitions.
That strategy deserves attention today because we're at a similar position today: The continuous, uninterrupted front lines of WWI and even WW2 (in most theatres and at most times) are out of reach in modern warfare. POur combat troops strength would not suffice; the whole NATO could not man the former Eastern Front as densely with combat troops as did the Wehrmacht even as late as 1943. Most conflicts would happen in a smaller theatre than that, but also with smaller troop contingents. Talk and writing about "empty battlefields" (on the operational level of ground war!) and huge "gaps" between formations has become acceptable if not normal years ago.
No matter whether we intend to be on the operational offensive in every future war - we still needs good ideas about how to set up an effective operational defence with minimum resources as well.
Modern military theorists need to adapt to such relatively empty war zones (as known from ground wars pre-1854), and the many historical precedents are msot likely of value for that effort.
The Limes Germanicus is an interesting historical example for a static linear defence at relatively low cost.