You want good fortifications? Look at some of the US coastal defense systems at major ports. The Manila Bay fortifications like Corregidor or el Fraile (Fort Drum) were quite good. Corregidor's 2 12" mortar batteries (8 guns total) had a devastating impact on Japanese operations up until one battery received a very unlucky direct hit on an ammunition bunker and was demolished taking with it the majority of the remaining ammunition available.
Fort Drum surrendered without being taken. It was so well armored that the Japanese had nothing to knock it out with (4 14" guns in near 360 degree traverse turrets plus a number of 5" and 3" guns).
In Hawaii and the Panama Canal Zone there were coastal defense guns to 16" sited in batteries that would have proved difficult if not impossible to take out. A note here: The US decided not to place these particular guns in heavy bunkers but rather on open 360 degree traverse platforms flat to the ground making them almost impossible to spot from the air when not firing. The crews and ammunition were in deep bunkers co-located with the gun for protection against counter-fire. This way, anything but a direct hit on the gun itself did nothing to take out the weapon and the only means to suppress it was to keep it under continious fire (a difficult proposition)
San Diego (Fort Rosecrans on Sunset Point for example where the guns were dug in deep within a cliff face overlooking the Pacific) and San Francisco bay were both heavily defended too as were many East Coast ports. These defense systems made what the Japanese had look like a joke. They made the Pas de Calais system the Germans installed look weak by comparison. Those 12" mortars were a particularly nasty proposition. They were usually sited inland, spread out for maximum defense against return fire in very small pits in pairs, usually on reverse slopes where anything less than high angle fire was impossible and, were invisible to shipboard spotters when firing. These mortars also fired a very heavy armor piercing shell that would impact at near vertical angles on a ship making deck penetrations very likely.....
One smart thing the US did was install a number of points along the coast and inland that were accessable by road for mobile 155mm rifle batteries that could be moved by truck. In this manner they could reinforce a threatened point with numerous batteries of guns in a short period of time.
For those of you into battlefield exploration a good number of these forts still exist to one degree or another and are worth a look.