Thank you so much for your comment! It was impossible to shoot inside the square, as it would be very easy to defeat the boss. I will definitely add instructions to the exercise.
Well, if you don't want player to shoot inside this box, not let the player to get inside then :)
You can just add second type of the wall that lets projectiles through, but not a playrer character. This will fix this problem better and will be more obvious for a player that invisible "no shooting" zone