I also don't know how to proceed with the mission of 'The Fortress'.
The road on the left eventually leads to this room.
Yes... and then you should have done a quest in there, a boss fight, rescued the Trapper Matriach and returned to her. And she should have given you a new quest, which is to raise your renown with the Trappers...
You'll be able to proceed with 'The Fortress' quest in time. For now keep continuing on with the Swamplands quests and it will wrap back around.