Garen's Wall is a substantial ridgeline located on the northern border of the kingdom of Ghealdan. It runs from the Forest of Shadows into the hills and mountains of northern Altara. One of the titles of Ghealdan is Defender of Garen's Wall.
The ridge is known for General Garen of Dhowlan, a nation of the Free Years which fought numerous border skirmishes with Farashelle to the north, before the time of Artur Hawkwing. Garen made expert use of the ridge as a defensive line, channelling the invading army into easily-defensible areas. The ridge was named for him.
A series of other conflicts have been fought around Garen's Wall from the Free Years to the New Era. It has been used many times to defend the country, at the present time Ghealdan, which lies behind it. Hence the ruler of Ghealdan has the title "Defender of Garen's Wall" indicating their defense of the realm using this natural barrier.
In the modern age, the ridge is located in mostly uninhabited wilderness, although on maps it is shown as forming part of Ghealdan's border with Andor and Altara.
Garen's Wall is depicted on the maps in the books. Robert Jordan explained its history during a Q&A on Tor's website. It should be noted that the officially-published maps seem to disagree on the ridge's precise location, some showing it lying only along the Ghealdan/Altara border, whilst others show it extending many hundreds of miles and deep into Altara, almost to the border with Murandy, although the presence of the Damona Mountains in this region in Knife of Dreams show this to be erroneous.