PixelJunk Monsters is probably my favourite tower defence game of all time. Castaway Island is basically PixelJunk Monsters - as simple as that. Usually I would deem this kind of plagiarism a bad thing, but goddammit I want to play PixelJunk Monsters on my PC at work!
It really is almost the exact same game, but I'm willing to forgive it this time!
Castaway Island [Armor Games]