So far I came across a website that shows some concept arts. It can be found here: Thief: The Circle -- A Through the Looking Glass Website
It’s not much, but it’s a start. Of course things like the street lights etc. need to be captured with a screenshot and then try to rebuild them. At least that’s the way I’d do it.
The Enemys in the first level are just regular guards, so no zombies or whatsoever. They look like this: Bafford Guard | Thief Wiki | Fandom
Also I found a table that points out the stats of the guards, when it comes to HP, Damage dealing, awareness, aggressiveness, and speed.
Gameplay can be found on Youtube from several people. I like this one, because the person playing is not talking through the game: Thief: The Dark Project -02- Lord Bafford's Manor [1/2] - YouTube