Because Grizzleheim gets progressively tougher as you go along, there are minimum levels to get into certain areas. You have to be level 30 to get into Mirkholm Keep, (I believe) level 35 for Nidavellir, and level 40 for Ravenscar. When you are the appropriate level for each area, talk to Bjorn Ironclaws to get permission to go into the next area.
