I think this is a great idea for a game, despite some drawbacks - protecting the portal over different platform levels with various monsters and challenges, not something I remember having seen!
I don't mind the graphics - it was fun noticing yellow pixels flying out of my machine gun, and who can dislike bombs walking on legs? :)
I was a bit disappointed that I reached the boss with only about half the weapons and upgrades, and there was only one boss... Survival mode provided a bit of additional fun for trying out the remaining weapons, but that's it. This game could really do with achievements and perhaps stars for upgrades, otherwise I only ever replayed levels when the next level was too difficult and I needed some money; a pity, because some levels which were difficult and frustrating could most probably be much more fun with additional upgrades, but I can't remember which level is which (thumbnail, maybe?) and have no incentive to repeat them.
Also, a small bug: when I got to the shopping screen usually one of the most advanced weapons was selected and described, probably because I clicked the same area in the game itself while shooting something...