Squaryd

This game is played on a 7×7 grid,with an empty space in the middle. Each player tries to place his checkers-like pieces (single or double) to form a square of any size with single pieces, while preventing his opponent from doing the same (blocking him with double pieces). He does this while avoiding forming a square with his own double pieces. The problem is that players compulsorily must place a double piece before playing any single piece!