I hear good things about U-Boat, though I haven't tried it, yet. I play Cold Waters and although it is not WWII and it is not a real submarine sim, it's kind of awesome and the sonar and torpedo mechanics are really well done.I need to see what cool, exclusive games there are on PC. For example, are there any modern versions of WWII submarine games with the look and feel of Silent Hunter? Oh, and Stronghold 2 (where you build castles) is an old favorite of mine - anything new in that regard? I guess you can tell when last it was that I played PC games, LOL.
As for building castles, if you liked Stronghold 2, there's... well... Stronghold 3.