Shield doors are Covenant or Forerunner created doors that allow people through while repelling plasma, gunfire, and virtually any other objects. They are similar in design to Bubble Shields with the main differences being the color and infinite usage time. Shield doors are similar to Anti-Vehicle Barriers.
They are also featured as forge items in the Heroic, Legendary and Mythic map packs, Halo: Reach, Halo 4, and Halo 2: Anniversary. They can be placed in almost any door as well as anywhere else the Forge user desires. In Halo: Reach, they can be one of three different sizes.
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 (through excessive use of 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.
- In the secondary versions of the maps Snowbound and Epitaph, namely Boundless and Epilogue, the map Shield Doors are removed.
- Several human structures have been shown to use a version of shield door technology, including the ONI headquarters in New Mombasa and the space station Anchor 9. Whether this technology is of human origin or derived from Covenant technology is unknown. Post-war UNSC ships also seem to have this technology in their hangers.
- 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.