I'm stuck in the first dungeon, the castle-like space near the strange man who sits and matches the 2nd card. (The entrance is in the screen to the left, with another cloaked NPC and statue. The other way out of the dungeon appears to be a few screens to the right, inaccessible as of that point.) I've entered a couple of rooms left (until the dead end), a few rooms up, and several rooms right, but i'm stuck.
I have one key, which is clearly for within that dungeon, because it only appears in my HUD when I'm in that dungeon.
There is one room, which I can only get a few tiles in (entering from the right), where I was in a small enclosed section with two of those red blob things, and some breakable floor. There are obviously other ways to get into that room, because entering from the right only takes me to that small enclosed part of the screen, I can obviously see the rest of that screen but can't get to those areas by entering the room from the right.
Hopefully, you'll know what I'm referring to without adding screenshots. The game is installed on my other machine.
I always look at the map very carefully. No "rooms" show up on it that I have yet to visit. There are a few open "branches" to rooms that I can't access yet.
Could you read my question carefully, and offer detailed help, please?
Your goal is to beat a boss and get a special key. Once you've done that, there is nothing left to do in the dungeon until much later. Now, I'm not sure if the key you're speaking of is the post-boss key or not, but assuming it is you should leave the dungeon and head for a locked gate in the area outside. I believe it's to the southwest.
If the key is a normal one, then you should look for a locked door inside the dungeon. As to where that door might be, I have no way of telling you unless I replay the game, which I'm unwilling to do at this point.
The joy of playing this game comes from the exploration, discovering the secrets of the world by yourself. Even if I could be giving you a detailed explanation, which I can't, it would ruin the fun.