Thieves' Guild is definitely the weakest of the Thief Gold missions. It is an annoying jumble of mismatched styles that drags on for far too long. It seems like it was assembled- poorly- from fragments of three or four other unfinished levels, and the divisions between those sections are both obvious and nonsensical. "Puzzles" involving levers controlling sewer hatches just seem to add complexity for its own sake, and the automap is worse than useless- it's actively misleading.

Coming right after Ramirez's mansion, Thieves' Guild merely adds more City underworld stuff without advancing the story very much. It's just more of the same, but worse, and the whole confusing two-hour slog strikes the player as pointless filler. The mission also commits a cardinal sin of Thief level design: the music in the casino is overwhelmingly loud and annoying, drowning out the environmental noises you use to orient yourself and stay aware of where the enemies are. For me, the only tolerable parts of the mission are the mansions of the two crime bosses. They're close enough in style to the successful Thief mansion heist formula that one can put up with them.

Taken as a whole, this mission is a confusing, pointless, and irritating jumble that lacks a coherent theme. It's probably the only mission in the original duology that I regard as a chore.

Good point about ambients. I think ambients in Thieves' Guild Demo release were much better.