Flash Circle TD
Flash Circle TD (thanks Sgt. Cilantro) - This link is terrible. It stinks. I hate it. Why? Because it ate up the last three hours of my night, which I should have been using to find an awful link.
Flash Circle TD is a Flash game (yes, really!) in which you build towers with various offensive abilities to fend off waves of increasingly difficult monsters who walk in circles through a maze, as monsters are wont to do. Figuring out a perfect build plan that maximizes each tower's effectiveness is a trial-and-error process, but it's definitely the good kind of trial-and-error.