CONTAINS SPOILERS

In order to unlock this area of the map, you must first head to Leyendell, Royal Capital, the large castle to the east of Atlus Plateau. You'll be given access once you defeat Godfrey the First Elden Lord (the major boss of the area). A gate will open on the main road from the Avenue Balcony grace which leads to Forgotten Lands. Reach Grand Lift of Rold to get to Mountaintops of the Giants.