Xel'naga Outposts were used as fortresses against any alien force that opposed them.
Appearing as towers floating in pieces, Outposts boast the most highly advanced weaponry ever developed by the Xel'naga (Starcraft II concepts: Xel'naga tower). Due to countless millienia of disuse, they were mostly dormant until the incidient at Bhekar Ro. Like the Relays scattered arcoss the sector, these structures are once again active and ready to serve anyone or anything that first come to claim them.
Housed beneath each structure is a matter-energy converter. When a construction unit of any race enter the Outpost, the machines can be reprogrammed to form larger more powerful defenses. For the Terrans, an Outpost transforms into a Proton Cannon capable of blowing apart capital ships in single shots. For the Protoss, it becomes a stronghold of psionic might called a Ziggurat. Finally, the Zerg infest outposts much like terran structures to create newer strains with unique abilities. After the transformation, the structure can not do so again and is no longer neutral to any race that comes by. Who ever claimed it first, permanently owns the upgraded structures until it is destroyed.