The "carpet" that covered the door was in fact a dense coating of moss. Carla scratched a little of it off with the tip of a manicured fingernail. The door underneath was made of what looked like tightly pressed wood fiber into which the moss was rooted. Carla puzzled over that a moment, but then realized she was looking at something that mages called a "living wall." The moss formed a natural, organic barrier through which astral creatures could not pass.