TLDR;
+ level design is generally interesting, albeit maybe less so than CotM
+ bosses are simple to figure out but very fun to clear the first time at least
- characters are not fleshed out enough to justify all the switching
- difficulty is too low, especially with revival mechanic
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Kurona's latest prank has turned the school into something of a demon's castle – an 'akumajou' if you will – and it's up to Shinobu and Maya to send her back to the HFIL.
Based off the demo, I had a more negative opinion of the game, but I suspected the level design would become more interesting as the game progressed. That guess was about right. It's not on the level of Curse of the Moon, but it's good, and I'm always happy to play a retro-style platformer.
Bosses also become more difficult, but I haven't lost a single life yet on account of the revival system. That's fine, I think the bad feeling of losing a character and consequently the boss regaining health is enough of a punishment to make you try to avert said loss. I would prefer if this was a little more... chewy? I like having to digest a boss fight. Curse of the Moon had more of this. Maybe this game becomes more difficult, but I feel like it's clearly going for a more breezy difficulty on account of it being a game partially for Gal*Gun fans.
There's a bit of a furore over the game not being a Metroidvania game despite some user-applied tags on the store page. It's fine by me, because I don't like Metroidvania games in general, but the approach to replayability here feels as if it borrows something from Metroidvania games as well as Curse of the Moon. That game used multiple routes that could be accessed by certain characters, sometimes only on certain playthroughs, as a means to make the game more replayable and also as a reward for keeping characters alive; the design was brilliant in both improving the game and adding some flavour, granting the levels an extra degree of mystique. Conversely, Grim Guardians grants you extra abilities, and multiple routes exist largely so you can explore and find more girls to rescue and more powerups. Unfortunately, it's not as holisticly applied as CotM, which would throw constant shortcuts and diverging routes, and it is no longer at all a reward for good play. Still, it adds replayability. It's FINE.
The two characters aren't fleshed out well enough to justify the character switching mechanic. I know the game is designed for 2-player, but what fraction of players is going to be playing this game 2-player? Are all the Gal*Gun fans going to be passing off the second controller to their girlfriends and wives? Seriously though. Both characters have one main attack, and a range of subweapons. Shinobu is a ranged character, and generally you will only use her when: 1) there are lots of ranged or aerial enemies; 2) you don't want to risk Maya losing life, since she has less. I feel like she would work best as a summon who stands on the spot and shoots forward, so you could get some synergy with her as you might be able to in 2-player. In fact, if they added that ability without changing anything else, they would have a more interesting game instantly. Maybe you could remove the switching and bump Maya's health, give her all of Shinobu's subweapons... I feel like this would also improve the feeling of the two sisters working together, it would add flavour to give the characters this kind of synergy and clearly it's something that is possible for the few Gal*Gun fans who have someone to pass off a second controller to, so it should be possible in single player too.
Graphically, the game is fine, but I keep fixating on the character sprites' faces. They look a little silly. Maya's face is particularly galling.
Anyway, this is good. Not as good as Curse of the Moon, but it's enjoyable.
Edit: after a few more hours... it's not bad, but the level design doesn't evolve enough and the enemies & gimmicks aren't used in interesting ways.
Also (gameplay spoiler) the second playthrough is not really different enough from the first. The boss fights have a few extra moves, but aren't difficult enough to justify the rematches. You get to explore more routes, but there is no general increase in difficulty. While we're at it the Kurona boss fight at end of the first run is just a big... what? Huh? That's it? Maybe it's meant to be braindead easy on purpose, but if so they went too far.