It's a really good idea for a puzzle game. However, there are still things to work on here.
First of all, moving with the character is annoying at times. Even when I'm not pressing buttons fast, the character still seems to have this weird cooldown, and, even though I press "d" 3 times, it moves right only twice. I don't know, MAYBE that's just me, but whatever it is, it annoys me a lot.
Exactly because of the above, I sometimes end up falling, which really makes this game way more unpleasant to play. Falling is generally not needed in such a game, it doesn't add up to the challenge, but rather frustrates the player when they press the wrong button... which happened to me QUITE A LOT of times, which gets me to the next point...
Having WASD for moving and ARROWS for building are really confusing controls, and I got confused many, many times and accidentally pressed the wrong button, of course ended up falling and restarting the level.
Treat the last the points not as separate, but rather as two points that TOGETHER result in a lot of frustration. First points is okay without the second, and second is absolutely skippable without the first (even though the controls would still be confusing, you wouldn't be unnecessarily punished for confusing them)
The graphics are overall rather good, though the doors kinda lack... everything. They are just this one-coloured, boring polygon without any substance to it (unless they have been opened).
The lack of any music is the game is really, really, REALLY pulling the game down for me, as music is generally the first thing I focus on. Even some really simple melody would help a lot (and I mean, that kind of melody would be best for such a simple, but still entertaining game). As for other sounds, they are okay, although you get bored of them rather quickly in my opinion, and they can get kind of annoying after a while, especially the sound of collecting a key.
And that also reminds me, for the amount of deaths you can reach here, the restarts between dying is a really irritating. Sometimes you just accidentally die, and you think "ugh, waiting for respawn again".
Also the tutorial text is both too big, and vanishes too fast. I had to die two times before I could focus on the text from block placement and read the important part of it, earlier I just got something about blocks and "up arrow key", but I had no idea what to do with it. I honestly thought I was supposed to jump over the hole, lol (which could be an interesting concept too, btw). I think you should just add some button for confirming that you read the text. Also, the text sometimes gets in the way of the gameplay :/
Another thing you could consider is changing "total keys needed" to the remaining number of keys you have to collect in a level. The total amount doesn't really tell you much after you've already started, and I think a lot of people, including myself, will just rush into the level without really planning what to do. After collecting some keys, you will just be left with "wait, how many keys do I have already?"
Alright, I think that's everything. Overall a really nice idea for a puzzle game, and it was definitely enjoyable, even though I don't necessarily like these types of games! Good work! :D