Look, I can give this game that it has some real cool design. Definitely someone has taken a page from design of Doom, since it has orthogonal unit differentiation (layman terms: each enemy is actually behaving different from each other, instead of just dealing more damage or taking more hits). However, I'd say game is still slightly flawed and could use improvement in some areas.
First of all, game is too chaotic. Game area is tiny, monsters and bullets are huge, and there's so much shit happening at all times! Like, my first introduction to the exploding demon bat enemy was when something I didn't even notice exploded into my face, and I thought what the hell was happening.
For the record, that is not a good thing.
Second, the game isn't superbly thought out in the way it presents its design to the player. Game is clearly inspired by bullet hell games, but hitboxes of bullets, explosions, and enemies are extremely vague and unclear - the only way to figure them out is by trial and error, which is hard due to chaos onscreen. Worst of all, your character moves extremely slow, and there are pond areas which make you even slower, which requires you to plan your every move carefully in advance - which is just not freaking possible considering that enemies can spawn literally EVERYWHERE on the screen, and half of enemies do a special attack on their death, so killing enemies isn't even always advantageous - and with clunky aiming and chaos onscreen it's almost impossible to aim your shots precisely to decide when you want to kill enemies.
Why did I encounter those? Because I freaking refuse to upgrade my HP - more on that later. In fact, upgrade system is one of the pointless, stupid ones where it's impossible to pick your way to play, instead you're expected just to buy whatever you have money for instead of deliberately picking certain upgrades geared for a certain strategy.
For example, in games like this I prefer to be extremely fragile, but fast, small and deal a lot of damage - and this is ALMOST possible! If you could upgrade your speed, and firing speed - since you already can upgrade your hitbox and damage - but again, upgrade system is geared towards you upgrading everything at the same speed, since upgrade price rises exorbitantly with each one.
Why is that a problem? Right now, there is only one way to play this game - the way developers intended. Have a lots of HP, tank a lot of hits, use spells a lot. And that's okay, that's decent. But the game could be truly good, or even great, if each player could pick what playstyle they want - for example, someone could be this slow tank with lots of HP that regenerates quickly and uses spells - but another might want to use only their gun, and be extremely fast and small knight with a huge hitbox.
That's what separates good from the great - when you look at every Deus Ex, Doom, System Shock, Starcraft, Street Fighter player - you will find that these games DON'T have a single optimal strategy, that each player prefers to deal with stuff in different ways. And that, to me, is what signifies a great game (or at least, game with a great gameplay).