Alright, I just beat the first boss/es.
I died about 3 times before realizing that there were a bunch of spectres around me. I died about 5 times before realizing that the rockets that I had been saving for the boss/es are able to damage me as well.
I probably died about 10 times before I came up with the strategy to kill the boss on the left quickly with 35 or so rockets and taking his little "house" hostage, Chaingunning all the spectres, and using another 35 or so rockets to take out the remaining boss himself. I was left with about...I wanna say 60% health left, and...I think 20% armor, give or take.
After this, I was kinda confused about what to do next, so I made an extra save, and after a couple minutes I found a switch in the right "house," and a minute later, I figured out what the heck it did.
I checked the map, and noticed that while I found the one secret in the level, there were about 15 or so enemies left(It's a feature of ZDoom.) Considering that I was nearly dead, and I only had about 18 or so rockets left, I was kinda scared that I'd have to restart the whole level over, kill the bosses again, and end up fighting 2 more of the same demon.. Thankfully that didn't happen, as I made my way to what I assume was a portal to...
*The rest of the post will contain early game spoilers. You have been warned!*
...hell itself. Several enemies spawned, and I pretty much was dead in 2 seconds. I got a message basically saying that if you thought you were supposed to get some cool ending for beating the game, you're wrong, because you still have 2 more sets of levels to go through(technically 3 since I'm playing The Ultimate Doom.) if I want to actually complete the game.
This is my guess for what happened. I beat the 2 big demons on the moon(I thought I was on Mars too. Odd.) in hopes of stopping all the legions of hell from arriving on Earth, then I ran into where the demons came from, hell itself. I was killed by the several legions that came out of it, and now I'm about to arrive on the entrance to hell itself!
I'm gonna stop for the night, but damn did that ending feel good!
Oh, and btw, I keep meaning to ask. Do you get anything for finding every secret on all of the levels in an episode?