Linear and easy, but fairly well-implemented.
What you've got here is a parabolic turret defense game with a single risk-reward schedule. That is, if you kill the enemies, you get points. If you don't, you die. Points can be spent in a shop to upgrade your gun or build defenses.
Rather than divide the game up into levels or waves of enemies, the action here is nonstop. At any moment, you may enter the shop to purchase upgrades. Which upgrade you buy first is a matter of taste, but you shouldn't have too much trouble cobbling together a winning combination.
Most upgrades have a significant impact upon gameplay, and upgrading a stat all the way makes a huge difference in performance. One caveat-- when you buy Max HP, it adds empty bar onto the end of your current health, so don't expect any benefit unless you can afford repairs, too.
The infantry collision bug in earlier versions of this game has been fixed... sort of. Shots still pass through the *body* of a soldier, but as long as they hit the ground modestly close to the target, the target will now die. No more crippling fear that putting more than one point into muzzle velocity will make it impossible to shoot basic foot soldiers.
There are still a few minor bugs floating around. The muzzleflash tends to get stuck on when you fire the gun too rapidly, for example. Sometimes a guy way in the back will die, seemingly at random, when you bomb a cluster of guys in the front. But for the most part, the worst, most crippling bugs seem to have been resolved. Clicking Upgrade while the Upgrade menu is already open, for example, now closes the menu. You no longer need huge splash damage in order to hit a close foot-soldier. I haven't been able to reproduce the bug where the turret build menu stops working, but I'm not sure what caused that one in the first place.
Strangely, I almost found the game more exciting back when it had bugs in it. Survival was harsh, back then, and you actually needed splash. It still leveled off to easy by the time you had all the upgrades, but at least there was a challenging bit in the middle there after the arial units started showing up where you wondered if you were going to make it to the next upgrade.
Now it's just static, linear, and very, very easy. By the time you've got even half of your stats maxed, even the strongest enemies in the game are no match for you. Mind you, I'd be *more* pissed off if the enemies quickly became impossibly powerful, and failure were the only option, but playing forever without losing is just trading frustration for boredom.
I'd recommend adding a definite end, for starters. Maybe the last enemy is a giant, slow mech-walker with tons of health, but he does a ton of damage if he gets close enough to fire. Then at least the game would have a point.
After that, maybe you could add some different scenarios, selectable from the map screen, in which different upgrade paths are secretly the most effective path to victory. That would extend the playtime of the game, but only if the player was likely to lose if they chose poorly.
Oh, also, if each upgrade cost more than the previous one, strategy while selecting upgrades would become more important. Just be careful. As with MMORPGs, the further apart each upgrade is, the more static the gameplay becomes, and therefore the more boring the game is.
Finally, if you do things that make the difficulty more challenging, please, *please* playtest it afterwards to make sure the game is beatable.
Thanks for giving the world another turret defense game. I love these. It still needs some improvement to make it a rich, complex game experience, but this is a good, solid basis for a game.