Good things:

• Excellent art direction; I love how everything looks. This feels like the game RavenSoft wanted to make in Heretic and Hexen (also, those health potions are a direct homage).
• Generally good weapon set (once you acquire them). The peat launcher is one of the best totally-not-a-rocket-lancher rocket launchers I've ever seen.
• Good movement feel.

Bad things/Nitpicks:

• Every single enemy except the crossbow skeleton and the Fiend/Pinky thing are easily foiled by strafing left or right.
• Ammo drop balance is wayyyy too weighted in favor of the shotgun crossbow. The game drowns you in darts before you even get either crossbow.
• What is the utility of the fire spell even supposed to be? The game immediately tells you that it's useless for direct damage, and if its intended usage is blowing up barrels, it becomes obsolete exactly as soon as you pick up a crossbow.
• Questionable application of checkpoints. One section has three checkpoints within ~40 feet of each other, while the section between using the turtle key and the library key has none at all for a good 30 minutes of gameplay. This isn't a huge deal if you aren't dying anyway, but it is a pretty jarring curve.
• Every weapon/equippable-item animation can be cancelled into a weapon change except for the purple potion, which forces a full animation before you can switch. I can see how it might be construed as gameplay balance, but right now it's just totally inconsistent with everything else.
• Inventory bar Tetris is kinda eh right now. I'm not a big fan of how dropping a multi-slot item changes the hotkey for every item after it. My suggestion would be for each slot to have a fixed hotkey and any multi-slot items can be called by any of the hotkeys associated with those slots.
• Current weapon progression is very much a slog in the first half, since you're stuck with a low-damage walking stick, a(n apparently) low-utility fire spell, and ammo drops for every weapon that you don't have.