From A Wiki of Ice and Fire
Greyguard is a castle along the Wall abandoned by the Night's Watch. It has largely collapsed. It has been abandoned for two hundred years. It lies west of Stonedoor and east of Sentinel Stand.
A Clash of Kings
Lord Commander Mormont planned to re-garrison the castle with men from the Shadow Tower, but on the advice of Qhorin Halfhand, they chose Stonedoor over it, due to Greyguard's dire straits.
A Storm of Swords
The party of wildlings that Jon Snow attached himself to descended from the Wall at Greyguard.,
A Dance with Dragons
Jon Snow decides to re-garrison Greyguard with thirty men under the command of Janos Slynt. Slynt refused to follow Jon's orders, the second time publicly, which led Jon to execute Slynt. Greyguard is eventually re-garrisoned with a grizzled steward from the Shadow Tower in command.
References and Notes