From SC2Pod
The Xel'Naga Tower is a new kind of structure introduced to the Starcraft Universe. It was first shown on Blizzcon 2007 in a Developers vs Fans matchup.
It is a neutral building on some multiplayer maps. If a player moves an unburrowed ground unit nearby (within 2.5 range) it gives a huge line of sight radius (22 radius) around it to the player and his allies. It does not detect invisible units. When two enemy units are near a Xel’Naga Tower, they both lose control of the Tower. Some summoned units will not activate the tower while others will, e.g. Infested Terrans will activate the tower while Auto-Turrets will not.
History
The Xel'Naga Tower used to be named the "Xel'Naga Watch Tower". This slight name change was introduced in Battle Report 4. The Xel'Naga Tower used to have a different model, which is visible in the screenshots below. The new model was first introduced in Battle Report 2.
Screenshots
