Rommel...D-Day

Not going tru reading all the comments above;but Isn't it obvious ?
In order to have any chance of success in containment of a numerically superior enemy it is essential to keep it trapped in a "bottleneck".
Be it a gorge, a small channel, or a chosen beachhead. ("this is SPARTA")

For the germans, the perfect defense plan would be to have more bunkers and pillboxes per mile in width and in depth AND to have mobile forces to reach the firehazzard beachhead within preferably 6 hours to mop up isolated paratroopers, punched thru regular infantry, etc.
Having a very tiny beachhead means congestion for the numerically superior enemy, meaning they can't deploy all their men and tanks and aircraft (!!) as much as efficiently as hoped for.
Whereas the germans could have their artillery harassing the beachhead and the floating troops at sea, far away from precision naval bombardment.
In return the germans would not expect to eliminate the beachhead without costs. Their tanks would not operate without heavy losses on the beach strip (under naval bombardment and airbombardment), and so would probably even won't show up at the immediate beach.
So at best it would be a bombardment attrition "battle" with likely 100% casualties for the ones inside the beachhead and the frontline around it.

The endresult: failure of invasion.

So technically it CAN be done.
But the atlantic wall in normandy failed another 2 years of dense and optimum defense projects, the counter offensive troops did not move (order Hitler), and when they moved they were constantly harassed by allied airforce and partisan sabotaging railways.

But what good is a supercommander if you don't have faith in your own defense project ?

 

The Japanese on their small islands (!! hence no reserves ...EVER) in 44-45 opted for a different approach to keep their men and defenses far away from the beach (more precise, from observation from naval bombardment )..because of these 2 reasons.
And 3d reason; battle objective is to kill and injure as much US marines as possible and to deny them the use of airstrip on that particular island as much as possible.
Different from the germans behind the atlantic wall.