I won't try to go into a big long thesis about your map design, just list some stuff that you should focus on for your next map or your next iterations of this map to help you get a bit more mileage out of mapping:
Your texture alignment is a little shotty, you should experiment with "unpegging" upper and lower textures above and below doorways/windows, the sides of doors when they are open should be set to lower unpegged, etc. Also if you're using Ultimate Doombuilder (which I recommend) get used to selecting a texture, using Shift+R and Shift+A (or just pressing A) to reset a texture's alignment, and then align all adjacent same textures automatically. Shift+A will align adjacent textures on both the X and Y axis, and A will align them just on the X axis. Experiment with these hotkeys they are really useful. Another point that might help as you're learning more is to try and make all your walls some power of 2 in length/height. You don't have to do this all the time, but when you're first starting out it can remove a huge headache of aligning textures if you're making sectors that are like 32 units, 128 units, 256 units, etc. Then when you texture the walls and Shift+R and then Shift+A everything just lines up perfectly. Definitely focus on experimenting with texturing around windows and doors.
Remember that doorways go from the floor to the ceiling, always. So if you don't surround your actual door sector on both sides with a ceiling that's ~128 units tall (or the height of the door texture you want to use), the door is going to look really weird and be way too tall. If you're confused about how to do proper doors there are a lot of youtube videos that can show you really clearly how to properly set up a door to look nice. Also pay attention to lining up door textures, again try to keep them the same width of your texture, and then Shift+R to reset the texture alignment for you. And unpeg the lower texture of the sides :3
One more thing is pay attention to where you give the player ammo and new weapons. I found myself basically at the end of the map with only a couple enemies left when I got the chaingun and the 40 shells, one way to fix an issue like that is you could put a monster closet that opens once you pass through that red skull door, but also remember that since that part of the map isn't really mandatory for an exit you might not want to just strictly add a bunch more enemies to the end of the map. Thinking about those kind of details really helps make a map feel better to the player.
One thing I found surprising for a first map is you actually had multiple instances where some good infighting takes place. The baron and 2x caco room was great, it let me save a handful of shells and I was able to easily get the cacos to infight the baron and then finish the baron with the pistol. Stuff like that goes a long way in maps.
Another positive thing is you use pretty organic shapes and room heights, I know I said earlier that using strict power of 2 lengths and heights can be good a lot of the time, but your location feels like an organic place, rather than a building or structure, so it was nice to see arbitrary room shapes, varied floor and ceiling heights, etc. Something that a lot of people struggle with when mapping is they tend to forget about varying floors and ceilings, not just between rooms but within the same room. Add puddles of water or blood that sink down a few units, add skylights in the roof, hell put some random floating cubes made out of mid-textures.
If you want my advice I would say go back and revise this map by starting from scratch and going room by room and focusing on improving each part of the map based on all the feedback you got, don't worry that it's not the most iconic layout or gameplay loop, just take what you have and focus on learning HOW to do the things better and make stuff look nicer.
