Ah yes, the trusty spiked wheels. Unfortunately they can't be deployed all the time, they might mark up the pavement." "But aren't they nuking this town?" "That's no reason to not be civic minded.
Protips: (1) Want to level up quickly? Dump 25-30 points into EXP the first chance you get. (2) You have to put 30-35 points into Speed to be able to outmaneuver most of the zombies below Wave 30. (3) The three weapons with Lethal Area stats are your best, best friends. Save up your money for them. (4) There is really no safe place to stand and shoot, so always make sure you take out the zombies closest to you, and always be ready to carve a path through the horde because in later Waves, you WILL get surrounded, and quickly at that.
My suggestions. (1) I know you were trying to pad out your game, but the price of the weapons is really, really high. It took me four or five 10-minute runs just to afford the second shotgun, and I can't even begin to imagine how long it'll take me to get the minigun. Which leads me to my next point. (2) From Wave 20 on, the difficulty curve shoots up way too quickly. It took me over 90 minutes of constant grinding to get to Wave 30, and from there, I just lost interest. (3) The Speed attribute needs to be tweaked. I had to drop 30-35 points into it just to be able to outrun the second-level zombies. (4) Would love to see more variety in the Achievements. Maybe like "Complete x Waves without taking damage" or something to that effect. (5) Autofire, dude. Nobody wants to get Carpal Tunnel just to play a flash game.
This is a very nice port of the NES game Fire and Ice. Why do I say port, instead of homage? Because I went back and checked, and they essentially ripped the levels from that game, and used different artwork. You get an E for Effort, but a 2/5 for a blatant rip-off.
Yes, I was inspired by this game (Fire and ice) and some moments have been transferred with improvements. It is not necessary to judge game being based only on the first levels.
I'm not fond of the way that the tiles are randomized. Out of five games, I had to restart four times because the tiles were arranged in such a way that would let me finish the level without using the hold tile function. Why even bother with the stars if your game makes it so that it's frequently impossible to three-star a level?
I know I'm not stupid, but there is no way to get either of the two keys I can see. I'm eight screens in, and I'm not going to go any further. Clearly, the designer of this game isn't doing something right, if so many people are getting hung up. Gaming 101, z3lf: Always have a clear, achievable goal for the player.