Well well... looks like a new community project happened during my absence. And for Heretic no less. What a nice thing to come back to.
So, now that I've spent my entire morning playing through this:
On the whole, it's quite a good project. Each level has some redeeming quality, usually many, and I enjoyed playing through all of them, whether it was because the gameplay was fun or because the level was simply good to look at.
Unfortunately, it seems I'm a bit late to this party to offer any suggestions for fine-tuning the design and texturing, so I'll cut straight to some potential gameplay/visual problems.
Jumping accessibility: There's a few areas that can be reached by jumping which probably shouldn't be. In most cases, it's not much of a problem. However, right near the beginning of E1M2, the exit can be immediately accessed by jumping from this ledge into the hole in the wall, allowing the player to skip nearly the entire level. Similarly, in E1M4, the exit can be accessed by jumping from this platform directly into the teleporter, allowing the player to skip over half of the final area. I realize the project wasn't designed with jumping as a focus of the gameplay, but you have to realize most people will be playing with a port like ZDoom, and even if you don't build it into the gameplay, you should at least consider preventing situations that jumpers can take huge advantage of.
Clipping Bug: In E1M3, there's an invisible pit to the left of this cage. Here's me falling into it, and here's me at the bottom of it. It's inescapable, even with jumping. I haven't checked to see if it's from bad nodes or incorrectly defined sectors, but there's no complex architecture that shouldn't be easily fixed.
Midtextures Through Floors: Again in E1M3, there are some midtextures descending very deep into solid floors. Here and here are two examples. Similarly so with some of the trees outside. A few pixels usually aren't much of an issue, but these ones going 24/32+ pixels deep just look buggy. Either custom-cut the textures or give the floors in the map a slight height change.
Also on the subject of E1M3, a small border sector with a lowered sky could be used to prevent this texture from jutting up into the sky like the inverse of the midtextures above. And then, while this thing looked nice, it quickly became a pain when I realized I had to run past it after pressing switches on the other side of the cavern to lower those lifts. I ran into it 6 times, missing the lifts every single time because of it, before I finally no-clipped my way onto the ledges.
Inaccessible Items: In E1M9, these items are placed in a small crevice that the player can't step down into. As far as ZDoom is concerned, this makes the items inaccessible, and the player will keep running over and over them but never picking them up.
Exit Switches/Floors Used For Non-Exit Areas: As a general rule of thumb, the deep blue teleports and wall switches are meant to signify an exit. It's been that way for nearly 2 decades in the original game, and almost every custom level has followed suit. Unless you want to trick the player into thinking they're taking an early exit, I'd recommend against using them as decoration or as standard switches throughout a level. E1M5 in particular did this with some exit gates in a secret near the beginning.
Anyway, those are the only big-time issues that jumped out at me during first play-through. I wish I'd been around earlier in the development for more alpha-testing feedback, but since it looks like you're trying to wrap it up, I'll leave it at that.
Nice job. Will probably play again soon.