Invisible walls are walls that are hidden from troops until they come close enough. When they do, the walls will slowly appear, but will disappear when the troops go away. They can still be blown up by wall breakers and earthquake spells. 25 of them will be unlocked at TH8, 25 at TH9, 25 at TH10, and 50 at TH11. Each will cost 50,000 gold to build and 75,000 to upgrade to level 2. There are only levels 1 and 2.