NoClip has put out a fantastic retrospective of the Thief games that interviews a lot of the creators on how the game was designed.
Edit: New things I learned (or had forgotten & learned again)...
- The whole magic system was geared around the 4 elements, which they later played down as such, but they also made up the original arrows, air/gas, water, earth/moss, fire... But noise & rope arrows were too useful to leave out.
- A major design idea was "active stealth", one idea of which was creating distractions and negotiating "safe spaces", and a major inspiration was Ken Levine's experience with sub sims, where you can release noise makers and use thermal layers to slip past, both of which had natural extensions in Thief gameplay.
- The other game LGS was making right at the same time as TDP was a Golf game. For that game, they had to fill a lot of dead space with commentator patter, which fed directly into using guard patter to fill time since the player was avoiding most things.
- I also respected Randy's postmortem of TDS. It was pretty clear he was just as crushed by the design flaws that the rest of the community was (the body awareness supreme backfire, the heartbreaking necessity to break up the levels to meet the XBox memory limits, the render woes & being stuck with a wonky engine, etc.), and I think he did a good job of explained how and why they happened. More or less, he made design decisions before getting all the relevant information that affected what they could do, and some decisions (like body awareness) just didn't work in practice like they may have looked on paper. I can't be too upset. Some good levels and moments still came out of that game, I still love Randy's levels & design thinking (generally), I can understand and very much sympathize with his position and (ultimately mistaken) thinking and intuitions, which I might well have gone along with myself if I were a team member, and above all ... if TDS hadn't been what it was, we probably wouldn't have Darkmod now, and I'm really, really happy we have Darkmod.