Hey, I played this. To be frank, it isn't a good level. However you do manage to do a few things well with it. I think it's an entirely serious wad, but it's also very much a "my first map" kind of wad. And since it is in fact your first map, that's excusable.
Let's start with the bad:
- Your level is basically flat. There was a small crack in the floor at one point, and some doors and switches, but that's about it.
- The map is extremely linear, it's really just one long winding hallway broken up by doors.
- The new textures look pretty bad, and the texture use in general is plain and ugly.
- The architecture doesn't match the textures at all. I'm talking about stuff like doors where the texture repeats one and a half times before getting cut off.
- Extremely lacking in detail. You've basically got empty rooms with enemies in them.
- A lot of very straightforward gameplay: open a door, shoot the enemies in the room, move on.
- Bloated filesize, like people said. Though really 6 MB isn't like a big file nowadays. Still you should avoid including resources that you don't use at all, where possible.
Now that's not meant to be harsh or off-putting, it's just a list of things to keep in mind for the future. If you look at other maps, even of doom2.wad, in DoomBuilder you can see how they use height variations a lot, interesting shapes, detail variations, etc.
Here's what was good:
- Surprisingly good ammo and health balance. A lot of new mappers tend to go to one extreme or the other, by giving you way more supplies than you need, or hardly any. It feels like you actually playtested this level a bit, which is a good sign.
- Similarly there's a good balance of monsters, you have a lot of them in there (almost 300!) and yet nothing is unmanageable. Again, new mappers often go to one of two extremes, having like two imps per room or going "lol here's 50 cyberdemons" and you avoided both these mistakes.
- While most of the encounters were bland, there were a few promising ones. The archvile setup was pretty good, the invul sphere fight wasn't bad, and the end with a whole lot of zombiemen to kill was a nice way to finish the map.
- Your textures didn't add much, but you got them into the map at least, which is a decent technical achievement.
- "Over here man" end switch was quite funny.
Anyhow, keep at it, having a map that works at all is a good first step. I think if you try to pay attention to what makes other levels look good and play well, and try to emulate that a bit, you'll be making some nice maps soon.