This is a Watchtower that is controlled by The Holy Nation situated in Okran's Valley. A few guards can be seen outside on shifts, on the inside, a good amount will be sleeping until you walk in. Inside you can find food, some Armor, weapons and of course Beds to rest and heal the wounded.
Given the location that it's built in, it likely serves as a place for scouting and fending off enemy military forces from United Cities to prevent them from advancing through Bast. Occasionally it may also serve to dissuade the Flotsam Ninjas patrols that attempt to break through the border at Northern Coast, and/or the Cannibal hordes that keep infiltrating the area of Okran's Valley.