CONCEPT
For those who don't know, "Thief: The Dark Project" is an immersive sim with a stealth focus that was released in 1998. "Thief Gold" is an extended edition that was released the year after. It's meant to be the definitive edition and includes, among other changes, three levels that were planned in the original but weren't in it. I've been told there's really no reason to choose to play The Dark Project over Gold, and I agree after playing it through and reading about the differences (even though Gold adds 'Thieves Guild').
You play as the thief Garrett (expertly voiced by Stephen Russell) in a dark medieval setting with some steampunky elements. Garrett is a quite cynical guy who is only out for himself. He has no special abilities (other than maybe good hearing and night vision but that doesn't manifest in being able to look through walls or whatever) and he’s not a fighter so you’re encouraged to sneak as much as possible. The blackjack and darkness will be your primary weapon. Guards hit hard and pretty fast but after some time you are probably OK fighting two of them. Three on you and you might as well quickload.
The story is presented through some fantastic stylized cutscenes, environmental storytelling, and scrolls/books found in the levels. It fits the tone perfectly and using the Dark Engine for any dialogue would be playing into the engine's weakness.
Now you might think you will only be breaking into mansions and prisons, but the story takes a turn. The undead is presented in the second level of the game, but it still takes some time to wrap your head around. After the tutorial and first level you are expecting something more grounded, but no. Things get weird. The undead enemies encourage a different playstyle since killing them is harder and knocking them out with the blackjack doesn't work.
The game has three difficulties. Normal, Hard and Expert. Changing these doesn't affect the enemies' damage or health but their numbers and gives you more objectives and sometimes changes the maps a bit so that new areas are available.

LEVEL DESIGN AND ART DIRECTION
The immersive sim principles used in making most of the levels are just fantastic. The levels are mostly open and logical which makes you feel like you're robbing a real place. There are rooms that are pointless for the player but exist because they are logical in the world of the game. This really gives you the feeling of exploring a real place that isn't "built for you" if it makes sense. The open-ended design creates loops in how you explore which often gives you a great feeling of discovery ("Oh this leads over here!"). The exploration is enhanced even more by the game's use of maps. You only have access to the maps Garrett was able to get his hands on and they are more often than not incomplete. One of the most memorable moments in the game was finding what I thought was a shortcut but I ended up in a mazelike surreal tunnel system with the distant sound of the horn I was looking for. I wondered where the hell I was and opened the map only to see "Where am I?" scribbled down.
You often only have a vague knowledge of where to go, like "the scepter is probably in the throne room". That's it. Where's the throne room? Well if it's not on your map you just have to look around. Oh, the door is locked? Well find a key then, where would you put a key? Or look for a hidden passage. Listen to the guards and see what they say. The conversations you can overhear with the guards are hilarious at times, but they could also drop important information. Just be sure to listen because the quest objective will not change and add this information for you. If you miss it you miss it.
Most levels do overstay their welcome a bit and by the end, you've probably neutralized all threats (except undead enemies you just have to dodge). The last 10-15 minutes of most levels were me bunny-jumping around trying to figure out the side objectives or collecting loot I've missed. It's at this stage where some of the levels can get really infuriating I'm sorry to say (I will never forgive 'Thieves guild' and 'Haunted Cathedral').
How's the art direction? Superb. The great use of light and shadow makes the locations look dramatic and intimidating. The sense of scale is really good.

GRAPHICS AND SOUND
Graphics. Well, they are what they are. The game can be really creepy and I think the dated graphics help make stuff scarier because your mind fills in the blanks in a way that modern realistic graphics don't. What else can I say? It's dated but that didn't bother me.
Incidentally, this game might have one of the best sound designs I have ever encountered. Not only are the atmospheres great in and of themselves (except the casino in 'Thieves guild') but the sound of footsteps is something else even though they are low quality. I don't know what it is but every sound seems like has a purpose. I'd say an unusually big part of the game's feeling comes from the sound design.

CONCLUSION
I loved this game over 20 years ago when I was a kid and now I fell in love again. I love it in that particular way that makes me not really care about its flaws, and there are a lot. The AI is dumb as a rock, limbing is janky, some levels are infuriating (I won't mention them by name) and sometimes the game just feels unfair to me, but I don't care.
I love it unconditionally because of its uncompromising attitude towards never holding my hand and just its sheer weirdness. It's one of the best (if not THE best) stealth games I've played. And it's just so damn fun!

If you decide to play the steam version nowadays I recommend TFixlite to fix the resolution and some other stuff
https://www.ttlg.com/forums/showthread.php?t=134733