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.
It happens, just steer it back if new stuff comes up.
Speaking of which, back on topic:
Invisible walls pose a bit of a problem with the AI of troops since troops will constantly have to re-target as new walls become visible.
Besides the fact that they will go around walls, re-targeting will slow them down significantly. Although we have visions of the delay of space (troop running speed) I think the re-target delay will be the most significant change.
The idea is that troops would target as normal until running into walls that the could not see. They then would redirect to a different target until running into walls they could not see. They then would redirect to a different target until running into walls they could not see.