Great game! It looks like it could have been made in the Doom engine, even though I see it comes with the UnityCrashHandler64.exe file.
The aesthetic was fantastic, really nailed the retro look. The controls were really fast and floaty (in a good way) like Doom, and the core mechanic was pretty fun. Huge amount of content too!
As much as I like the retro-style "hard game" aesthetic, I feel like at times went from "hard" to "unfair." The first level jumped really quickly in difficulty without much explanation at all. You can basically get 1-shot instantly after you open the wall after the first skeleton if you pause for even a second to take in the new scenery. It also doesn't really make much sense for the first enemy in the game punish you for using your melee attack - a move you just learned. I think I would replace the white skeletons in the first area (the area with the pond in the middle) with blue skeletons. They're fast and do lots of damage, but they at least feel fair for a person just picking up the game. I would also either give the player access to up/down controls for the whole game rather than granting that as an unlock, or at the very least remove most of the slime enemies that appear before you unlock that ability. One slime is a lose condition by itself with no mana (which in the first level, you should really expect a player to have), and two slimes can wobble you with basically no counterplay even if you do have mana. That's much less "dark souls" and much more "ninja gaiden," which I'm not really that into. If you have up/down controls though, you at least feel like you can fight back. Checkpoints would have been nice too, given the difficulty.
That said, I think the bosses were both pretty unique (and I did beat both of them ;) ), and even though they were riddled with slimes, they were at least predictable in the first boss, and manageable due to the expanded controls in the second boss. I really liked the red necromancer enemy, since it takes some time to figure out the best way to beat him, and once you realize he can generate mana for you if you leave him alive, it puts you in a really fun risk/reward scenario that I'd love to see more of. Grouped white skeletons were also a treat to see when they spawned in, knowing one good shot would take them all out. The first boss did this really well with a combination of white and blue skeletons in a clump, and slimes behind them. Trying to fend off the blue skeletons while angling a good shot around them to take out the white skeletons was the right amount of difficult and fair to keep me going through several tries.
I also like the cheeky use of browser links (How did you get a link to this page before you submitted the game?), and the switch in the forest level got me pretty good.
Really great game, and impressive amount of detail. 9/10!