Pretty good but needs some work.
You should add a better background and lower the prices on weapons. Plus, consider shortening the length of the later levels. I spent like 5 minutes on one level mindlessly shooting easy type enemies and didn't have enough gold to get anything.
Rather Simple
There a quite a few defense games on the portal, and this one doesn't particularly stand out from those. A simple graphical interface, with nice, but still simple gameplay. Point-and-shoot no longer can get a good score...add some customization perhaps. Things like interchangeable backgrounds or extra music would pull away from the too-simple gameplay.
However, if worked on and expanded, I think this would make an interesting addition to the Defense section of NG.
Really well done
You, Stu Cooke (51lver) have made a masterpiece although it can be improved by having a completable mode,achivements and a wider range of enemys. I look forward top seeing Shape defense 2, or maybe there wont be one. Up to you.
Too simple
As you probably know, the idea isn't exactly anything new which is a major problem. It is has been done many times before. Would have been more interesting if you could have moved your cannon around and added in power ups etc etc
The graphics were simple, perhaps a little too simple, but I did like the explosion. The blood wasn't terrible but I feel you could have gone a little easier on the filter (you can keep the filter but rather than making the 'shine' a perfect white just make it a lighter red and I would have thought that would improve it 100x straight off). It would have also been nice to see more violent screen shakes when you got hit by a bomb.
All in all, not a bad game just a little too simple for my liking.