After being introduced to the cabin and the ash circle, I had initially thought Ben (or someone else) had the ash circle to keep Jacob prisoner. However, after the Season Five finale, I think it is the opposite. That the ash circle was to protect Jacob from the smoke monster. Think in biblical terms, God is Jacob, the other mar in Lucifer. The smoke monster is a tool of the other man, to repeat vegence, etc. Somehow, JAcob banishes the otherman, and in doing so, brings smokey under control. But to control smokey, Jacob has to keep smokey from turning on him. Jacob insulates himself by having this protective circle. Its a bit abstract, and I am sure Rustytrombone will critize it, as he usually does my posts.