PC Gamer have published a retrospective article on Ultima Underworld: The Stygian Abyss, a game I certainly have fond memories of from back in the day. Here's a snippet:
It takes some getting used to, particularly if you haven't played an older game like this for a long time, but after a while it's manageable. Use the keyboard controls for moving and the mouse to turn, and you're golden. I was soon rediscovering the sense of sheer exploratory joy that's baked deep into Underworld. I mean, bloody hell! Underworld's eight massive levels, each almost a game in itself, are proper 3D environments, with angled walls and floors, and ledges, chasms and lakes. You can run and jump and swim, and even levitate. You can look up and down, for God's sake. There was also genuinely groundbreaking texture-mapping, which transformed the way videogame graphics looked forever.
As well as being genuinely exciting technical innovations, all these things made Underworld a fundamentally different experience to the then-standard '˜move one square forward/back/to the side' affair. Most importantly, it gave you the sense of being in a real environment, not just a grid-square map of locations. For the first time you had to take into account vertical architecture as well as horizontal, while the ability to jump and fall let Underworld introduce puzzles which required you to properly explore the space. Fantasy battles could be epic affairs for the first time, where you leapt about on tables and ledges to get a height advantage not just you standing there bashing the Attack button and waiting for the enemy to take their turn.