I were immersed by this game. Good introduction screen, but it was too fast for me that explanation on the screen was 2 seconds and game began to play. So, I handled it after the second try :D I was happy that there weren't any bounces with random direction. I think you need to show players where is allowed to place walls and where is not. A little help from UI won't disturb the main feel of the game. Speeding up is also made good.