A Moongate during the Third War.
A Moongate is an enchanted, magical fortification, usually prominently featuring a gate composed of Ironwood, metal, or another resilient material. Moongates are also framed by stone, wood, or metal reinforcements.
While the Moongate is primarily a Kaldorei innovation, the High Elves made extensive use of the design in their outposts. Perhaps the most famous Moongate in Azeroth is the Gate of the Three Moons, a gate that once protected the entrance to the heartlands of Quel'thalas. The Lich King utlized a number of magical keys to open the Gate of the Three Moons, which led ultimately to the defeat of Quel'thalas by the Scourge.
They are so named for the fact that the original Night Elven Moongates were enchanted with the light of the moon itself. Subsequent High Elven versions utilized the Arcane as a substitute. The enchantment imbued upon these structures makes them practically invulnerable to traditional methods of siege, and they must be unlocked with magical keys or deactivated with equally powerful magic.