not bad for a draft

Just like any other td game i've played in the past i might add but i don't care. Always enjoy a good TD game. But i dunno if this is a bug but i did play it through lvl 50 and once i tried to go to 51 it stopped. Seems like probably your cut off point but figured i'd say something. Cudos on the game hope it turns out great.

Mechanics ok...

Ok, I appriciate its a draft version, but obvious stuff: 1) Sound, lack of. Needs fairly good effects to make up for basicness of graphics, possibly a slightly different bang depending on type of gun/explosion etc.
2) Instructions/tutorial for those who have never played TD before. 3) Nice title graphic on Normal/Hard selection. Also, to what does 'Upgrades on right cost 500' refer to? The actual upgrade button costs whatever the tower requirement is. You might be better putting the cost next to the buttons themselves 4) Mechanics are good for the most part, but in later waves speeding up locks up the whole game for 5 mins while it sorts itself out. 5) Balance needs adjusting, theres no real need to ever get new tower types, or even upgrade the ones you have, all i did was buy rows of starter towers, getting a new one virtually every wave and most baddies did not make it past the first corner. 6) A bit more zing in terms of, maybe announce Bosses coming, or countdowns to special units arriving or something?
Other than those point, operates smoother than mant TD games, I look forward to the finished version.

okay

I see this is still in the early EARLY stages, and it's therefor to pinpoint any specific problem. It works, that's about all there is to say. As the player, I'm wondering how to unlock some of the secondary abilities you have shown (Homing bullets, prediction, etc) You should try to make the enemies a little unique, other than just different color renderings. Sounds and music, perhaps some particle effects, and a little more detail to the grid would be nice. It's a basic TD game, and you get points on succeeding in that area, at least. I also couldn't figure out how to unlock the other towers.
All in all, it's probably too early to ask for feedback. I know you're already working on some of the problems I've mentioned, so it would be better to get some of the basics sorted out before trying to find any smaller bugs in the game. I didn't see any, but as the title stated, this is your alpha stage. Best of luck to you!

A good start

Everything you already put in seems to be working fine, with one small exception: it looks like the circle that's showing a tower's line of fire doesn't use the tower's center as it's own center.
For the final version, be sure to add this:
A way to see how far a tower is already upgraded at a glance.
Up the difficulty, right now it's way to easy.

lacks tutorial

I can see what you were trying to do here, i can see it would've been good if you had've explained how to play the game. im playing it here and i normally like tower defence games, but this one is just confusing. i have no idea what to do in it. i'm sure if i tried i could figure it out, but thats an unnesescary inconvienience for me and for other people. graphic)s were ok, there was no audio, which maybe kills it a little, (but not that nesescary) and gameplay is confusing due to lack of tutorial. but anyway, what saying is i appreciate the effort put in, but improve. definitly put in a tutorial and if you could, maybe an audio.