Invisible walls are solid, invisible surfaces that block the movement of objects. They are often used in situations where normal terrain is inadequate. to prevent players from exiting level boundaries or performing various other glitches.
A related concept is the kill barrier, a typically-invisible surface that kills players (and sometimes other objects) on contact.
Properties of invisible walls Edit
Running headlong into a normal wall in some games will result in the player model being knocked back; the same property may not always apply to invisible walls. A player model running into an invisible wall will often simply slide along the wall, maintaining a running animation even if it is moving at a snail's pace.
Some games, in addition to using invisible walls, also use more intricate, "friendly" setups. In Legend of Zelda: Wind Waker, for example, sailing to the edges of the Great Sea displays a short message before automatically turning the player around. Halo: Reach gives players a ten-second countdown (and a warning to "return to the battlefield") when they cross "soft" boundaries; failure to comply results in the player's immediate death.
Halo 3 is particularly notable for its intricate -- and perhaps excessive -- use of invisible walls and kill barriers. Some of the game's barriers will repel a player with magnetic-like force. Should a player successfully bypass a level boundary, they will usually be killed instantly by an algorithm that checks for objects outside the map. Assuming that a player bypasses a level boundary and ends up in a location not checked by that algorithm, the player may still die if they touch the ground outside of the map for a total of 5 seconds -- and it need not be five seconds in a row. Only a small handful of locations outside of a map can be safely walked on.
In Halo Custom Edition, invisible walls in a custom map can be made to distinguish between projectiles, players, and vehicles.