I was right up beside the door and it made a noise when I tried to use the key but it was like a noise to signify that it was wrong, and it didn't unlock the door, so I still couldn't open it. And in the video the player unlocked it with a copper key. So none of that makes sense. I can try again to be sure but I really think I did it right. If it still doesn't work I'll just have to give up though. Are the next two Thief games like this as well? If so then I might skip to the fourth one, because I think it seems more linear and easier to figure out. Don't get me wrong - I like free exploration and figuring out what to do involving a lot of different stuff in a large area, but in this case it's just a huge headache with way too many problems getting in my way. But does the fourth game have any story continuity with the previous trilogy so that I wouldn't know everything that happened before, or is it more of a remake/reboot?
You're banned for worrying about the copper keys, I assure you that door wasn't important. If you try it again and you do find a copper key, that's great. Doesn't it say what the silver key is for? The throne-room probably, that's where the target is.
Thief 2: The Metal Age is near identical in optimization, art-direction, and design; although 2 remains more loyal to stealth because the late-game enemies are robots instead of demons. (And the deadliest robots can be easily tricked into killing themselves if there's a wall within a foot of their right arm-cannon when they see you.) Thief 3: Deadly Shadows improves the graphics so that the frame-rate on my sissy desktop is a barely tolerable 15 fps and there's a slowly expanding hub-world in the form of city streets. Both maintain hand-drawn maps and 3 introduces little challenges to collect some of them, but 3 also replaces the lock-picking to be more involved and eliminates your trouble with keys entirely.
Thief 4's optimization, story, and hand-holding are all trash and a total disgrace to the series, but I might yet give it a chance just for being a playable stealth game that discourages murder. The details are irritatingly fuzzy on how it connects to previous games, but a Youtube commenter claims the new Garret is born centuries after the first and just happens to also have a young girl side-kick just like Garret obtained at the end of 3, so it seems plenty obvious it was originally meant to be a sequel and they made last-minute rewrites like making his right eye magical instead of (spoilers).