When I go up to it I get the two options of 'claim this for yourself and attempt to rebuild' or 'leave this place alone'. After accepting the quest, getting around 45k stags, 3 sets of tools and 70ish men I set out and eventually found the 'ruined castle' in Essos. It's the Old Valyria quest you get from Ser Janis Lyren in the crossroads Inn.
