This game gave me mixed feelings. But let's start from the start.
The graphics and music have cohesive style, and fit each other. Zombies look cool, screen shake, when the giant comes is a nice effect. The subtle animations on the buttons are something I always forget to do, but show how polished the look and feel really is. Enemy variety is a big plus.
But the game economy is seriously off! The only thing I could ever afford was ammo, so I didn't upgrade at all. Then again I didn't need it, because not counting the one time a fast zombie caught me off guard, the basic cannon worked just fine.
My advice? Make the ammo free, and the game harder. Maybe make the cannon overheat or reload after six shots, so the player doesn't spam fire all the time.
The second thing- I don't understand why I can choose the room to be attacked. I'm always gonna pick the one where I have the cannon and ammo. And not to be nitpicky, but it makes you have two rooms in a jam where the theme was "one room" :)
Over all- a stylish game that needs rebalancing. Still- good job!