Citadel of Adun
The Citadel of Adun, or Citadel for short, is a building warped-in by Protoss as a prerequisite for the Templar Archives. Additionally, the Citadel researches the Zealot Leg Enhancements upgrade. The Citadel is powered by the Pylon.
The Citadel does not build or unlock any units directly, and is thus often regarded as a one-time use/stepping stone building in climbing the tech tree. Its Plasma Shields can be upgraded at the Forge.
The Citadel is typically built in all race match-ups, with its Zealot speed upgrade one of the earliest upgrades to be researched, along with Dragoon range.
In the PvT match-up, the Citadel is sometimes warped in at a chokepoint or to form a wall at the natural base, alongside Pylons. This is generally because the Citadel is not seen as a particularly crucial building after the Zealot speed upgrade has been researched; as long as the Templar Archives has not been destroyed, the Protoss player would still be able to produce High Templars/Archons and Dark Templars/Dark Archons if the Citadel is lost.
Leg Enhancements significantly increase the movement speed of Zealots. This upgrade grant Zealots the mobility to effectively engage units that it otherwise could not, such as Siege Tanks or Hydralisks.