Mediocre
Now, the concept of using Pokemon in a tower defense game is very cool, and a great idea. However, the game itself leaves much to be desired.
The graphics are very bland, consisting only of A) ripped sprites from FireRed/LeafGreen and B) Times New Roman text and ugly-looking boxes with nothing but primary-colored gradients to give them any sort of color.
The music is also ripped straight from the original Pokemon Red/Blue. It's good music and fits the action well, but the high-pitched chiptunes get really annoying after a while.
The story scenes are poorly-written and, in my opinion, uninteresting. You can skip them, of course, but it'd be better if I didn't WANT to skip them.
The game gets too hard way too fast. The first few levels are a cakewalk, and after that, there's only a very short period of minor challenge before it becomes extremely difficult.
It's quite impressive that you managed to include all 151 original Pokemon, but I don't really think that they were all necessary. Many of them have identical attacks from others, just with different names.
It's not all bad, though. I like that you can only use a total of six individual "towers" at a time, and that the design forces you to constantly move them around the arena. Considering how many tower defense games there are out there, I don't really know if that's an original idea, but I've certainly never seen it before. And though, as I said, it's a little too hard, I'm sure one who plays a lot of tower defense games will love the breakneck pace of the action.
And, of course, you are still in the Alpha stages. So I hope that in the future, you can improve this game tremendously! It's got great potential!