Conceived of by a Covenant defensive mastermind named Zurghed, a Zurghed Wall is built thick enough to house a garrison. In the Demon Spawn War, these types of walls were needed to deal with magic-using demons with the ability to turn a section of wall to mud or pass through it like a mini-rift. With a network of chambers in the wall, they allowed troops to close-off the attack or at least contain it with series of sliding blocks and stout doors.
The insides of Zurghed Walls are compartmentalized to protect against gas and fire attacks, and to sealed off from other areas. The cost of making these types of walls is often so much in labor and magic, that only the greatest of cities and fortresses have them.