In the very first level, in the same courtyard where you pick up the flare pistol, there is a small cemetery area. And in the corner of that, there is a door that appears to lead into some sort of crypt or mausoleum, but if you try to open it, Caleb just says something to the effect of "aw, its stuck!". I never figured out to get that door open, or if there even is a way to get it open. If it is possible to get inside that building and anyone happens to know what it is, I would greatly appreciate the help. That damn stuck door has been haunting my dreams since 1997 (not really).

I don't know about getting there, but you could always use noclip cheat (if there is any, of course) to satisfy your curiosity :)

Yeah, but if I do that and there is something in there, like items or whatnot, then it means there is a no-cheating way of getting that goddamn door open and that is what I really wanted to know.
EDIT- there is a noclip cheat (Eva Galli, in case you're curious) and I just tried it. If you use that cheat and walk into the door, you just end up on the top of the building, which has a Guns Akimbo powerup sitting on it. So seeing as the inside of the building doesn't even exist, I guess that door is just a gag meant to haunt the dreams of high school juniors well into their late 20's, lol

You mean this area here, when you have to jump to get the double weapon item?
http://www.youtube.com/watch?v=JqY81O6TYoY#t=0m22s
I don't have the game installed to try it, but try what klaymen said, use the noclip cheat to be sure that there is something behind it.

Well then, nothing there.
It was only placed there so the curious player can jump up on it, and get the akimbo item as a reward. :p

There is a narrow sector looping the inside of the chapel set to teleport the player upwards, so you can't cheat your way in. It is only deactivated once the door is opened by regular means.
I'd tell you what's inside but it would be too bad to spoil one of the coolest secrets in the game.

Pretty much just detail showing off the "true" sector over sector with the advanced Build Engine.
Or just let your imagination run wild.. maybe there is a hidden item on the map that will allow you to unlock the door and Jojo the Idiot Circus Boy pops out with a shotgun.

Thanks. So this is the only super secret for the first level? 11 regular, plus 1 super secret?
I'll buy Blood again from GOG, this time I'll go after every secret. :p

No, I was actually just quoting an old troll from back in a day when BLOOD used to be fresh and numerous questions about this particular tidbit were relevant.
Sorry, I just couldn't help it. Made myself laugh a bit and made you decide to buy the game, hopefully, so it's a win-win?
The actual super secret of the level is a very obscure arcade puzzle in an entirely different location.

LOL, trolls just love me.
I would buy it, anyway. :)
I was 90% sure that there is nothing there, as I've used the noclip cheat back in the day... and yes, I just ended up on the top of the building. But I did left PredakingCrush to try it on his own, maybe he could find the right spot in the building, if there is one.

I've got all the secrets on that level and that's definitely not one of them, sorry.

Right, there is nothing in that building and no way inside of it. I turn on the clip cheat and walked into it from every direction and angle and no matter what, I always just ended up on the roof.

I guess this is why they don't add many fake doors in FPS. People keep wanting to know what's behind them.

lol