Robots vs Robots is a free-style (open-env) tower defense game, and it's free.
Ranked in these QuestionsQuestion Ranking
Pro Great design quality
The towers and robots are nicely designed which show a nice polish to the title.
Pro Free to play
Free with no In-app purchases, though there are advertisements.
Con Not many maps
Only 5 different maps to play really limits the longevity of the game.