Rampant Games' Jay Barnson took their re-release on Good Old Games as an opportunity to write a retrospective blog post about the two Ultima Underworld games, in which he analyzes what the titles did right and how they measure up after all these years. Definitely a recommended read. Here's a snippet:
But besides the technological innovation (which doesn't age well), Ultima Underworld (and the sequel) did a lot of things very, very right. The ability to trade with most non-hostile NPCs, for one thing, was really cool (duplicated somewhat in the first Fallout games, but not so much elsewhere). The more open-ended (simulation) approach to the adventure was really refreshing and enjoyable. There were a few puzzles that demanded (or at least suggested) specific solutions, but much of the game allowed you to organically problem-solve. Maybe that capability seems exaggerated through the rose-colored glasses of nostalgia that I look through, but that is how I remember it. This was long before anybody coined the term (sandbox game,) and I don't feel it was too literally that kind of game. But if you are a fan of the Thief games, that open-ended approach to missions originated in the Ultima Underworld series.
In many ways, I prefer Ultima Underworld'˜s approach over the common sandbox approach which feels like you are just encountering the random dungeon inhabitant of the week. (Huh? What makes you think I'm talking about The Elder Scrolls?) As I recall, the world, treasure, creatures, and everything are fixed (and therefore, to me, feel more meaningful) every game begins with exactly the same. But every game also plays out differently according to your approach (and, sometimes, just random events in the game).
The designers also captured the claustrophobic feeling in both games. Both games had you trapped in an underground prison in the second game's case, the castle had been surrounded by an impenetrable blackrock shell, and the only escape was to similarly confined worlds also controlled by the Guardian. In the first game, you encountered cultures that had adapted to living in a massive prison. In the second, you saw glimpses of the psychological toll of the imprisonment on the inhabitants of the castle over time.