I think it's worth wondering whether we would know about them. A fort doing adequate deterrence would likely not be assaulted. So e.g. the Ligurian Wall was never attacked as far as I know, but then it was essentially useless, since the Allies opted for landing in Sicily and Southern France. Would they have landed in Northern Italy if it hadn't been for the Ligurian Wall? Unlikely (Liguria looks like the perfect place to mess everything up during a landing), but not impossible.

I think that forts were a bit like a strongbox in a bank: they aren't meant to resist against all that may come, and they can be forced open with enough preparation and effort, but they also are good enough to raise the bar of preparation and effort to a point where an attack becomes much less likely.