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.
a Spanish colonial general who fought Napoleon's invading armies
In pre-mechanical times it was impractical for communications and armies to travel effectively across Europe.
the Sahara desert on the outskirts
The king's invading armies were huge oppressors in the captured countries.
WW2 began with Hitler's armies invading Poland.
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
Invading Armies
The First Battle of the Marne.