Play Poland. Fortify Wloclavek, Plock and Katowice. Pwn Hitler.
Fortresses are good when you know or suspect you'll be fighting a defensive war in an area, where you won't be able to attack for a long while. Naturally, that area must be yours from the start if you want to fortify it. This was the consideration behind the Stalin line, the Maginot and Siegfried lines, Eben Emael, the Sudetenland forts, the Atlantic wall... you name it.
Now, the above might sound like a Captain Obvious moment, and maybe it is, but the problem is that we, as players, can always find solutions to attack, to seize the initiative, to take land. I'm not that good at HoI, but, playing Poland with almost no buffs at all (just a few modified and extra specialties on the tech teams) I can eventually beat back Germany (which is, in my personal mod, buffed rather more than Poland) and begin to take their land. Hell, I even manage to attack Allenstein though as a rule I can't really keep it in the first year.
If I minmax at the level some players on this forum have shown, I can probably either nail down Allenstein easily or actually keep North-Western Poland (Poznan, Gdyna etc) and grind down the East Prussia natural pocket.
With that in mind, building forts makes no sense, because I need to put all those IC days toward hitting Germany harder. The game is conceived in such a way that the desired result, pwning Hitler, is best achieved by trying to seize the initiative from Germany rather than trying to create an airtight defense and outlast Germany. The offensive approach would have been unthinkable in real life, and it is unthinkable against a human Germany (in fact, if the GER player is at least at my level of skill, it's altogether impossible to beat back Fall Weiss as Poland).