Shield doors are Covenant, Forerunner and human-created energy fields that allow people through while repelling plasma, gunfire, vehicles and virtually any other objects. They are similar in design to Bubble Shields with differences being the color and infinite usage time. Anti-vehicle barriers are a larger-scale application of shield door technology.
Several human structures have been shown to use a version of shield door technology, including the ONI Alpha Site in New Mombasa and the space station Anchor 9. Whether this technology is of human origin or derived from Covenant technology is unknown.
There are several in past Halo games and they appear in Halo 3 on the multiplayer maps Epitaph and Snowbound. In the secondary versions of the maps Snowbound and Epitaph, namely Boundless and Epilogue, the map shield doors are removed.
If one stands exactly in the shield door's energy field, doing a melee or shooting in any direction will be negated by the shield. However, the player is still vulnerable from attacks from either side. This also works with the Bubble Shield. By overloading a map with shield doors (Using excessive Trip Mines to cause the map to lose control of the maps graphical and geometrical assets), the shield doors on a map will disappear.
- Despite the fact that shield doors protect from projectiles, plasma and vehicles, explosions can still affect players and objects on the other side of them. A perfect example of this is the underground shield on Snowbound, where the nearby power coils can be detonated to instantly kill players standing on the other side. It is unknown if this is a glitch or Bungie meant to do this.
- The shield doors in Halo 3 all appear to be of Covenant origin and have obvious projectors while the Shield doors in Halo: Reach appear to be of Forerunner origin and appear as a simple wall of energy with no obvious projection point.