Hmm...that's a difficult one to judge.
You see, on one hand it was way too easy. Every gun, other than the one in the first level, makes killing the zombies way too easy. The most difficult part was to find a zombie to kill to progress with the game. But this also made the game fun. Like, sure, overcoming difficulty is part of most games, but just going ham on zombies without a care in the world, is nice, you know?
A big problem was that the Zombies didn't chase after the player unless you got really close to them. But since all guns(except the level1 gun) was long range, there was never a need for that.
Most disappointing was that you put in the effort to crate all these different guns, but they are only randomly given to you at the start of a level. Picking them up with limited ammo in the levels themselves would be better i think.
I never had to use the close combat weapon nor the bombs, and i don't know know what the two bars on the top left are for. I assume one of them is health, but the other? Never changed for me. Got to level 12 until i quit, as having to kill more zombies meant more time aimlessly searching for them. Instead of simply making the player kill "more", the enemy should instead be more difficult to kill with each new level. At least in my opinion. But don't simply give them more health either, instead make the A.I. better with each level. Something like that.
As others noted, not being able to see the bullets was a tad confusing at first, but since i used a tablet it made no real difference to me.(i assume this game was made with a mouse in mind, right? I think i had a slight unfair advantage)