Razor Wall

Razor Wall

Seen completely surrounding the Citadel and certain areas of Nova Prospekt, these bluish black "walls" are actually a type of vehicle. This is proven by leaving and then returning to City 17, where the mobile walls are in a new place they initially were not. They are designed to crush any nearby material by expanding outward and up, which is assumed how the Combine are able to harvest a nearly endless supply of raw material. Little else is known about it.