The Shadow Tower is a castle on the Wall. It is located close to the western end of the Wall. It is one of three castles still manned by the Night's Watch at the time of the series.
The Shadow Tower is one of only three manned castles left on the Wall along with Castle Black and Eastwatch-by-the-Sea. There are nineteen castles along the Wall, but as their numbers fell over the centuries the Night's Watch had to abandon sixteen of them.
When Jon Snow infiltrates the wildlings, Orell asks him which of the nineteen castles along the Wall are still manned by the Night's Watch. Jon admits that only Castle Black, Eastwatch-by-the-Sea, and the Shadow Tower are still manned.
In the books
In the Song of Ice and Fire novels the Shadow Tower is under the command of Ser Denys Mallister, an experienced and conservative commander, and uncle to Lord Jason Mallister of Seagard. It is not the western-most castle on the Wall, which used to be Westwatch-by-the-Bridge. However, Westwatch has fallen into ruin and is no longer garrisoned. Qhorin Halfhand was usually based at the Shadow Tower.
According to Jeor Mormont, at the start of the books there are about 200 soldiers in the Shadow Tower. Qhorin led 100 of them to the Great Ranging.
Patrols and scouts from the Shadow Tower have to keep a close eye on the surrounding terrain. Though extremely difficult, it is possible to by-pass the Wall by descending through the Gorge and crossing the Bridge of Skulls, or by tracing a dangerous route through the Frostfangs to the west. The Night's Watch has to keep an eye on these routes for signs of wildling activity.