The guardhouse was used by Tuck and Valaraukar during the war between Mirith and Andris as a supply depo. Andris lost control of the guardhouse after a larger battle on and surrounding the bridge. Tuck and Valaraukar managed to flee to Andris during the battle.
To get to the Andris Bridge from Mirith, one must exit town through the East Gate and take the southeast path at the crossroads. That path leads directly to the guardhouse. To get to the bridge from Andris, simply exit the West Gate and follow the path. Also, one can travel directly south of Mirith's East Gate and cut through the forests in order to reach the bridge.
The Andris Bridge is sometimes used as a way to get to Skull Island. If one travels due southwest of the bridge, they will run into the island eventually.