The Greymane Wall was erected after the Second War, by Genn Greymane, as the nation of Gilneas isolated itself from the Alliance due to a dispute surrounding the internment camps for the orcs. It is a large wall with a portcullis, located at the southern edge of the Silverpine Forest (near Pyrewood Village) and the northern edge of Gilneas.
The Greymane Wall is the only evident site of Gilneas. This enormous gray wall marks the boundary between Gilneas and Lordaeron. No guards stand at its battlements or man its gates — the wall seems dead. Nevertheless, refugees from the north camp in the wall’s shadow, hoping for succor. These unfortunates never see a Gilnean and are easy prey for the worgen that occasionally stray this far south from Shadowfang Keep. ( )
Those on the other side in Gilneas have been spared from the Plague, though the worgen curse has taken effect instead. Many refugees that have escaped the Plague have made camp outside of the wall, but its gates have yet to open, presumably because the Gilneans are also suffering a devastation.
In World of Warcraft
There is currently no substantial terrain beyond the Greymane Wall, merely a relatively large grass expanse surrounded by steep sides.
Oddly, as seen in the top left-hand corner of the picture of the Greymane Wall, the wall seems either never to have been fully completed or to have suffered at least an attempted invasion. This might, however, also be a result from the developers reusing the model of Thoradin's Wall, which is identical to the current in-game model of the Greymane Wall.
The humans surrounding the Greymane Wall are approximately levels 18-21 and the beasts around it are approximately levels 16-17. In addition, there is a neutral weapons vendor named Wallace the Blind.
If you stand in a certain "invisible stair" on the right side of the wall, you can clearly see that there is nothing behind the wall. There is simply a path that increases to a grade of 1 foot off of the ground, then ends.
If you try swimming from Hillsbrad Foothills to get behind it, you will find that the map is innacurate. Most of the peninsula behind the wall on the map is just more sea.
This section concerns content exclusive to Cataclysm.
In the expansion, World of Warcraft: Cataclysm, the Greymane Wall received a major graphic update to accomodate the architecture of Gilneas. The gates of the Greymane Wall have been shattered, revealing what has happened to the kingdom of Gilneas and its citizens. With the Worgen curse taken hold, they have found a partial cure, allowing them to retain their Human minds even when transformed, and allowing them to transform at will. Venturing forth from Gilneas and seeking help from the Alliance, they have decided to rejoin them, to combat the new threats of Cataclysm.