The answer is the North European Plain.
Because invading armies would need to cross them and they would help prevent that from happening.
Invading Armies, get Nappin!
Army of Jericho.
In the postclassical era, which roughly spans from 500 to 1500 CE, various groups conducted invasions across different regions. Notable invaders included the Vikings in Northern Europe, who raided and settled in parts of the British Isles and beyond; the Mongols, who created one of the largest empires in history through their conquests across Asia and into Europe; and Muslim armies that spread Islam through military campaigns in North Africa, the Iberian Peninsula, and parts of Asia. These invasions significantly influenced the political, cultural, and social landscapes of the regions affected.
a Spanish colonial general who fought Napoleon's invading armies
The king's invading armies were huge oppressors in the captured countries.
the Sahara desert on the outskirts
WW2 began with Hitler's armies invading Poland.
In pre-mechanical times it was impractical for communications and armies to travel effectively across Europe.
Europe was through history a very disputed continent. Natural border were clear and simple to agree, and much easier to defend against invading armies.
Ullyses S. Grant was the general of the Northern armies (Union) who went on to become president
The First Battle of the Marne.