Hey - nice prototype, it has a cool bouncy feel even at this stage which I think is maybe a good indicator that you'll be able to add a fair amount of juiciness to the later alpha build.
I like the initial feel of the controls, though I kept on getting pushed into a wall then losing the ability to move - not sure if that's me being bad at the game or a bug :D The shooting is cool though, has a satisfying feel even at this stage. There's obviously loads more you can build on in that respect. If there was a tonne of different explosive weapons firing off everywhere, and enemies exploding and spikes...spiking...etc, then I reckon you can definitely use this as a basic prototype that delivers a fair amount of fun for what it is.
I could obviously say the visual stuff that needs to be improved, but as you've stated this is a prototype there's no point in me saying "oh you need better gfx" but I'm definitely intrigued to see what they become - could be anything from random bugs with guns on their backs to actual tanks haha :D
Anyway, my only major issues were the collision detection on my character, like I say I'd have a bit of a shoot-out, ping a load of enemies around, then get sandwiched against them and the wall and then the controls seemed to stop working so maybe I was just destroyed, which actually makes sense now I think about it haha. If that's the case maybe need more info on that for dumb asses like me ;) Also, I didn't get the slammer thing too well, maybe that needs more explanation and ease of access as stated in other reviews. I do like the idea of having that on the right mouse - again, for dumbasses like me, and then it'd maybe make it more fun when it's easier to implement, cos otherwise a shootout is my first instinct and I tend to ignore the slammer.
Hope that makes some sense and maybe helps you a little bit in developing it! Great work in a short time and interested to see where it goes ;)