Xel'naga temple

The Xel'Naga are a race in the StarCraft universe which have vanished (or had gone to sleep) countless ages ago. They have visited many planets in the Milky Way Galaxy, including Aiur, homeland of the Protoss.

The Protoss Temple on Aiur was built on a nexus of cosmic energy (the same type of energy used by the Dark Templar and the Zerg) and marked the spot where the Xel'Naga first set foot on Aiur.

It was destroyed by the Zerg so the Zerg Overmind could manifest itself physically on Aiur. The Overmind said of it: "Behold, there is a temple, not far from here, that lies upon ground most hallowed. Though I have born witness to the passing of countless millennia, the temple which you must assault is older by far. For it was constructed by my creators, the Xel'Naga, and it marks the site where the Xel'Naga first set foot upon Aiur."

The second Xel'Naga Temple was found on Shakuras, current homeland of the Dark Templar. Again, it was built on a nexus of cosmic energy. It consisted of a pyramid-shaped structure with a small floating crystalline top, and appeared very different (and larger) than the Temple on Aiur. Through the use of the Khalis and Uraj Crystals, Artanis and Zeratul were able to use the light and dark sides of Protoss psionic energy to activate it, and scour the Zerg from the dark world.

The Xel'Naga Temple did not have a psionic shield during the game, but it did during the cinematic as the Zerg attacked it after it was activated. Most likely the shield could only activate when the Temple itself was activated.

Theories about the Temples
Very little is known about the Temple on Aiur, but the second Temple had a few clues:

First was the use of cosmic energy. It appears the Xel'Naga, a race which had at least some psychics (and used psionic technology such as Khaydarin Crystals), but this energy is opposed to the "purity of essence" used by the Khalai. This is the same energy used by the Zerg.

Through study, the Dark Templar knew that the Xel'Naga Temple could destroy the Zerg, possibly through the use of cosmic energy, but why was the light side of Protoss psionics required as well? Possibly the Temple was supposed to be used by Xel'Naga and Protoss working together to attack the Zerg. The Temple only affected Zerg on the planet, though.

The Temple was not reactivated in Drawing the Web but could be reactivated in Enslavers: Dark Vengeance, which took place before Drawing the Web.

Some fans of the game claim you can see "bugs" in the Temple. While the Zerg are "bugs", these bugs didn't resemble any known Zerg Broods. No one knows what the Xel'Naga look like, but it's likely they are not humanoid in form (since they're not pure of form, and the Protoss are humanoid in form). It's possible these "bugs" were representations of the Xel'Naga.

Trivia
In the older, pre-release version of StarCraft, the Temple was a Protoss structure which created Protoss Archons. The building was removed from the tech tree when the method of creating Archons (merging two High Templar) was changed.