The Citadel

It's so obvious dude was making a ryona fetish hentai game and halfway through he was like "wait a sec, I have a pretty good shooter on my hands now" and switched course midway. And I love every second of it.

The horrific anime girl violence sure is a vibe but this is actually a really fun shooter with good level design and music. It's worth the few bucks it costs if you can stomach the, ehm, aesthetics.

Pretty fun boomer shooter with unique gunplay mechanics, especially if you grew up with the OG's. Lots of reviews call it a "gooner shooter," but those are probably the same types who never watched Ghost in the Shell or Elfen Lied because they think even a minutia of exposed skin means people are gonna blow a load like they're Mount Vesuvius from the exposure.
I you enjoy good character/sprite designs, or a "show, don't tell," story format, this is also a good choice. If you enjoyed, be sure to check out the version 0.4 demo of it's sequel: Beyond Citadel

This game was deep in my memories. It seems so obscure, but it should be renowned the world over. It has everything I like: Guns, anime women, religious imagery, horrific gore, deeply involved gunplay, platforming, upgrades, sweet characters, and one of the most unique, efficient, and breathtaking graphical directions out of any game. There's some minor bugs, and I recommend adjusting the controller settings in steam input. After that, the game is a thrill to beat! The "easy" difficulty is at least as hard as classic DOOM's "Ultra Violence", so starting there will give you a very satisfying climb up the difficulty ladder.

starts off slow since you can't quicksave so unless you have extra lives any death sends you back to the start of the stage, which wouldn't be bad if the game starts you off very underpowered
once you do get more upgrades and weapons the gameplay is very enjoyable and the art is nice (even if it is a little unsettling since im 99% percent sure the anime girls getting gibbed is the dev's fetish)
easily recommend if you like anime and like boomer shooters

cool tactical shooter game but it does suffer from enemies being really far away and super high up just pinging you + the not at all subtle over saturation of guro fetish content

Playing the sequel before this one was a risk but in the end I don't think it affected my impressions in any negative way. I would dare say it made me realize that the sequel might did some things worse. But let's take things from the start.
The game is a hybrid of boomer shooter and a more modern tactical one. You die very fast and even though you can rush things and clear entire rooms if you want, usually it's better if you take your time, assess the situation, pick your weapons carefully and hey, when required peak around corners and thin out the enemy numbers before entering an area. Adding to the tactical element is also the manual reloading where if not careful you can run out of ammo during combat and stuck with an empty weapon and unable to attack. You can slowly reload each gun but an enemy can charge and kill you before finishing the animation, or if you are in a rush you can throw the half-empty mag and instantly put a fresh one by tapping the reload button twice. It's a cool and simple mechanic (fossils like myself might remember this from the classic Lucas Arts game, Outlaws) that forces you be more accurate and better in managing your weapons and ammo.
Level design is for the most part pretty solid, mixing levels where the focus is on careful platforming, wide areas where you need to keep moving and avoid enemy fire and close quarter maps where you carefully move from room to room peaking around corners, slowly making your way towards the end. Most levels are pretty small and you can finish them in a few minutes if you don't die since dying will send you back to the beginning of the level with the exception of a few ones that have a checkpoint you can return to granted you have extra lives.
Shooting in The Citadel feels amazing and it's probably the best attribute of the game. Every single weapon feels powerful and can be used through the entire game. Even the melee weapon will turn most enemies into gibs and explosives can turn an entire room into a mess of body parts, glass shards and blood. Some levels get so absurd that the game outright crashed on me due to the utter chaos weapons can produce.
Visuals is probably the weakest aspect of the title. Sprites are well drawn and I love the eerie and otherworldly feel of the world, but at the end of the day every single level uses the same colors and very basic assets making most levels feel similar despite the varied level design. Personally I didn't mind, I think especially in the first game the level design is so well done in most cases that I barely noticed, but I can see how some might get tired of seeing the same colors and shapes in almost every area.
Sound wise, probably the center stage is taken by the combat sounds. Death cries and enemies gibbing sounds are borderline excessive and exploitative and most weapons sound as powerful as they should given the devastation they leave when using them. Music is pretty decent as well and fits the atmosphere of the title.
Before closing I should address the fact of a lot of people complaining about the violence in the game. All enemies in The Citadel are basically cute anime girls and even though all of them are behind masks and armor, blowing them up and seeing them in pieces on the floor can be a bit too much for some, like the more exploitative anime of the 90s and 80s. Personally I didn't mind and I don't think it ever goes that overboard with the gore.
In general, I highly recommend The Citadel. It's a great shooter, at a low price with distinct aesthetics and solid level design. Despite the sequel having some improvements and adding a bit more depth to the mechanics and storytelling, I think this title is in ways better. The level design is more solid (the sequel has way too many levels that are in open areas with enemies attacking you from across the map with annoying accuracy), and in general I think it flows better. Still, I would recommend getting both.