I'm playing Knightin'+ and I think I'm stuck. I'm on the 4th dungeon. I have found two keys, but I have already used both of them. There is a third door which requires a key (in room 4-2 on level 4-2, accessed via the teleporter at room 1-1), but I cannot seem to find the third key (I think I've double-checked every room on every floor accessible to me, and the only unopened chests are currently blocked and I don't think there are any unsolved sokoban puzzles.
So you've chosen the Crimson Armor route. The first key that you can obtain is on the floor 4-3 in the room 4-1. The second one is on the floor 4-2 like you mentioned. If you collected both of them you'll have to buy the key in the shop. The boss route is pretty straightforward. You'll be able to find one key on the each floor starting from the floor 4-4 except of floors 4-6 and 4-7 (there are no keys on these two).
Thanks for the quick reply! Your response encouraged me to persevere and I have managed to make some additional progress now. (Specifically, I previously somehow missed the instructions on level 4-5 about how to activate the lanterns, now that I've figured that out more rooms are opening up). Anyway, thanks for making this game, it is really fun!