Pretty good. Had some real issues with fps when you start to burn in the sun for me, otherwise. Its a solid little tower defense. My only issues would be the Ui being so big., maybe having some way to hide it when the mouse goes over it might be a quick fix? or a button that lowers it or raises it when you need it. Getting rid of the box around all the various health bars and reducing it to just the hearts would help a lot too!
Still, good little game! great job!