Most important possible fortification in the 20th century?

IMHO if you posit using nukes, fortifications once again become important. Sure nothing short of being under a mountain will save you from a direct hit, but even Maginot level fortifications with some modifications will allow survival and function unless a direct hit. The problem is the terrain. If you have rugged terrain and choke points, fortifications can do a great deal - this is why many fortifications of antiquity and the middle ages were built where they were. Someplace like Poland, Ukraine, or the American Great Plains, unless you build a chain of mutually supporting fortifications fortifications are much less useful and building on that sort of scale is expensive to say the least.
Something to consider is that point defence systems can make direct hits with nukes much harder to achieve. Won’t prevent an airburst, but the direct hit will still need to get very close, making the PDS job easier. Though it’s the whole thing about exchanging toughness and mobility. You would need something that makes the investment worthwhile, that prevents the other side from using theirnown resources to go around/above the fortress to strike the rear.

You need to reinvent the choke point in the era of air and space power, which is hard.
 
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Static fortifications became significantly less important after gunpowder and especially so after the end of WW1. However they still obviously had strategic uses.

What's the most important place and time that a particular line of fortifications could possibly be in the 20th century? How long could an individual line of fortifications be held during a single war? This could occur during any conflict in the 20th century.

If the Munich Agreement hadn't happened and WW2 started in 1938, would the Czechoslovak border fortifications have been considered this?
castles in the sky aka the chain home Radar network.
 
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