So the textures and character sprites looks great. The game feel is good, and moving, jumping, and shooting all feel right. But the level design is cruelly unfair. Level 1 is totally fine but,
Level 2 - Before you're even able to realize what's going on, you fall into a pit. If you kill all the zombies in here, the only way out is to reset your game. Very cruel trap to put at the beginning of the game, especially when you're still trying to figure out if there are any hidden mechanics for how to get out of there. But even if you do manage to jump over the pit, you're blocked from continuing without an obvious way forward. With nothing really telling you to do so, you're supposed to jump into an alcove to trigger zombies to come out, which you then have to shoot to continue. And the way the zombie spawns all of a sudden means you'll probably die to him a few times before figuring out where the trigger is. Then, after going through a brief corridor and falling down to a platform below, you have to fall one more time to reach the bottom of the screen. But sometimes, a zombie might be directly below you when you fall, and the camera doesn't pan down until you actually start falling, so you have no way of being able to detect the threat ahead. The other rightmost zombie actually looks like he was made to not move until you hit a trigger that's near him, which is probably how the other zombie should have been, but the rightmost problem has a different problem -- an invisible object prevents you from shooting him (presumably a hidden zombie, who absorbs bullets when invisible but can't be killed or kill you). This is frustrating to the unknowing player because it feels like this zombie has an unfair amount of health or something and just won't die, and you end up running into him despite shooting a whole bunch of shots at him. The secret exit thing is fine though.
Level 3 - You jump into an arena area, where you are now bombarded with too many zombies to be able to kill reasonably. The only way I found to handle this is to hide in the 1 square alcove that you can go in, but the two square tall zombies can't. Then you have to wait patiently for an opening and tediously shoot and kill all the zombies. The idea of hiding in the alcove to solve a combat puzzle is kind of neat, but not when it's so easy to die from execution of moving left and right and shooting, and when it takes so long to defeat all the zombies. Then there's a trap on the left side of the level, which is cheap, but not totally unfair -- after all, the door down there is gray and there was an indication of another path at the beginning of the level. But dropping down to the secret area sometimes has that same issue as on the second level, where you have a chance of falling onto a zombie that you aren't able to see before falling in.
Level 4 - Eh, it's short and isn't that bad if you know what you're doing, but since the lives seem like they're bugged (perhaps you're losing 1 life for every frame you touch a zombie, even after you die) it means one mistake in any part of the game and you have to do everything over again. And this level gives you practically no time to think. I'm thankful for the brief invincibility period when the level begins at least.
Anyway, cool art and game feel though.