Belgium was bristling with defenses in WW2. In the same way France, Poland and the Western front of Russia were. They had networks of expensive forts with numerous gun turrets on them.

eg. https://en.wikipedia.org/wiki/Battle_of_Fort_Eben-Emael

Wars tend to expose obsolete military doctrines. Big fortresses on land and sea (battleships) that can bombard targets many miles away are great if the enemy doesn't just fly in.