The walls are built like the shells of an egg around the precious yoke. The precious yoke in this case is the Purple Flower sect.
A single shell is strong in its own, but more eggshells are built around the egg in order to reinforce it. So the more walls there are, the more difficult it will be for the egg to break.
The job of the people of the Purple Flower sect is to reinforce their sect. This job is very important because if the war goes badly, the grounds of the sect will be the final battlefield.
But despite how important this job is, only the people of the Purple Flower sect are allowed to do it. They are not allowed to receive any helpers in order to avoid the infiltration of enemies.
For the Purple Flower sect, only exit is allowed, not entry. And this exit will only happen when the war is about to start, and they have to join the battle and defend the walls.
