It's a Devil Daggers clone, but it's pretty well done for what it is. It isn't hard at all, at least to get the achievement which is the closest thing to "finishing" it. You run a circle in an arena while zombies and hive objects that spawn "bats" (Similar to the skulls in Devil Daggers) appear. You have to destroy the hives or the bats will overwhelm you. There are also werewolves and what look like mental asylum patients fumbling around the map. The mental asylum patients don't move so you have to jump over their attack. The werewolves are by far the most annoying enemy in the game because they charge at you so you can never run in a straight line.
Eventually you face harder versions of these enemies, and then at around 265 seconds a final boss appears. Really, you only need to survive to this point because the boss clears the map for you, but the map becomes so cluttered with enemies you pretty much have to take most of them out. In particular the hives because they have only one weak point. You can dash, and this is super important, far more important than jumping, since it will make you essentially invincible if you always dash into an open space.
The crossbow handles well, but nothing is explained. Killing enemies drops "souls" and unlike in Devil Daggers the powerups never disappear. You can level the crossbow three times and also do a power attack by pressing "Q" on the keyboard. The issue is the bats are going to flood the screen so much when you turn around you won't even know what you're shooting at, but you should always use the powerup on the enemies trailing you since it ensures survival. If you hold down the right mouse button, you can aim your crosshairs and shoot an explosive attack. This is useful for clearing the weaker mobs trailing you and for targeting the eyes of the hives. Also, if you double click the LMB you will shoot a shotgun-type blast. This is WAY more useful than in devil daggers since most of the enemies have a moment where they are immobile and you can blast them to death before they have a chance to react.
There's no AI, really, the enemies just trail you, and the final boss is a joke. All you have to do is shoot the eyes until they bust open. Then, the game cycles on repeat forever, making the scoreboard virtually useless. There is a learning curve around 130 seconds, but once you get past that the game is all but won. I did it in 15 hours, but I have OCD and wanted to have a perfect run where I cleared the entire map before the boss appeared. It's good if you want a "Devil Daggers for Beginners" type game, but as its own thing it does ultimately feel kind of pointless after a few hours.