the wall disappears some battles and is there some battles because when the enemy pillages you, there is no wall, whilst the enemy attacks you, you have a wall
it doesn't really matter too much, since all you need to do most of the levels is use the crossbow, then once you get 9999 mana, start spamming units and you win
I wish the gameplay differed a little. Do you really want to play maybe 50 levels that are about the same thing and beat them over again 2 or 3 times each to get a glowing frame? Fun game, but gets boring once you see how many levels there are :(