Castle! Castle! is a prototype for a cancelled game in development for PC and Gamecube at F4 (formerly known as F4-Toys), a French video game developer based in Paris composed of former staffers from Adeline Team and No Cliche, including Frederick Raynal. It seems that it was going to be a real-time strategy game / tower defense, where the players would have had to build and protect their castles, but sadly the project was cancelled. This could have been an evolution of another F4 proto, know as Pocket Castle.
Thanks to Jay for the english corrections!