Invisible brush-entity which allows players to climb on its surface on touch. Player won't stop using the ladder unless jumping off or moving away from the ladder's surface sideways. Ladders on ceilings work. Sloped ladders produce dissatisfactory results and should not be used. Touching a ladder while falling will nullify the player's velocity and safe him from falling damage. Usually used in combination with a func_illusionary with ladder-textures for the visual counterpart.