There's a hub with 4 exits in the first Catacombs 3D game. You'll know it when you get there.
One of the only places when you can get stuck, because you're required to solve a puzzle. There are sufficient hint scrolls in the levels themselves. But there's just one meta thing to remember: don't forget which sub-level has a hallway of doors, when you see it.

I got all this, but I found myself stuck when I got to level 19. I always come up one key short and have to go back to level 1.
Any idea where the extra key is hiding? I know it's somewhere because I used to play this game back in the early 90s, and I recall finishing it.

If you mean the key for the hall of doors, I was under the impression that there aren't enough to open all the doors, so you have to experiment, memorize which ones have what in them, and skip certain doors.

I thought you had to identify which door was the correct one in order to progress (don't all others take you back to the same place?). As long as you get the scrolls you should be able to work it out. Two attempts was all it took for me to work it out, but I did collect all scrolls.

Protip: you don't need kill boss in Nemesis Lair to win the game, you can get to Nemesis Workshop with Gremlinar if you take left route behind the wall and find correct path in labyrinth (lots of blood demons).

Hey SCPM.
Thanks for the reply, but no I don't mean that one. After the hall of doors, where you take door xx to proceed to the mysterious 12th tomb, you're placed in a level where there's a mirror back to world one at the start "wimpoids exit here", a bunch of smaller dragons, one optional larger dragon in the "dead end", and an "altar room". I always find myself at that point without the key needed to get into the alter room, where I can see one of those invisible mirrors, which I'm assuming takes me further on in the game to the final 10 levels.
I've shot every wall in this level a bunch of times, yet I can't get past this door. I vaguely remember the extra key is hidden somewhere else, and you can start this level with one key, but I have no idea where to find it.

It seems you're referring to the first title of the pack. You can find the key - I assure you. If you're really stuck you can check out the map through this site (that's how I found it!): http://www.vgmaps.com/Atlas/PC/index#C

Ah yes indeed. Sorry, I must not have read the initial post correctly.
Thanks for figuring out what I actually meant and pointing me in the right direction.

Glad I could help