Most can be opened directly like a normal door but some only open when you hit a lever or pull a chain or whatever nearby (for those you'll be told it's locked in which case you'll know to look around for the thing that opens it).

You should see the bald patches that I still endure today from trying to get old dos games to run functionally. :P

Guys, I can't thank you enough. I mentioned UW2 in the wishlist thread a few months back, and here it is! Finally I can replay a legitimate copy of this brilliant game.

Those games run pretty well under DOSBox.

Thanks for ye olde recommendation for Arx Fatalis :). I actually heard of that one before, I might get it one day, but not too sure. The main character has to be cool enough for me to play it. In Ultima Underworld I could put myself in the game, I can't with Arx Fatalis.

About secret doors in Ultima Underworld, it's rather funny, I ended up finding a bunch just randomly looking for them. I'd hold the right mouse button down, then move the mouse in a downward motion to open the door, and I'd keep doing that on the walls, and.... A secret door opens! Not the best way to find them, but haha, it works! ;)