Does this game not have music?
I like the strategy part, that you have to choose when to stop making walls. You want there to be a lot of walls for a high score but not too many walls that the level is impossible.
One suggestion is that if the level becomes impossible (you used all your bombs and can't reach the exit), the game should tell you that.
Another suggestion is to tell the player something like "press any key to stop generating walls", otherwise they wouldn't know what to do at the start.