The Outer Elfgate was guarded by the Elven Ranger Corps, which needed to be eliminated in order for Arthas' passage. The Inner Elfgate was magically sealed. It could only be opened by the Key of the Three Moons. The Key of the Three Moons could only be forged by combining three different Mooncrystals. These Mooncrystals were in possession of different high elf forces. However, Arthas managed to destroy all these forces and create the Key required to reach Silvermoon.
In World of Warcraft, this area appears to be associated with either the Thalassian Pass (which connects the Ghostlands with the Eastern Plaguelands in-game), or the Dead Scar which follows the general path of the Scourge as seen in Warcraft III.