The Last Expansion
The frontier is where the last great imperial expansion took place about a 100 years ago.
Having already displaced local population during their previous western expansion, they were met with greater resistance. The hill people were reinforced by Karakos, the Federation and the Kingdoms of the Great North, all of them seeing The Empire's advancement as a threat.
Thus, The Empire had to eventually sue for peace. They then created the Free Cities as a buffer, to protect imperial peace.
Each territory in the frontier is ruled by a lady or lord. Like in the rest of the empire, the ladies and lords are granted these rights in a temporary, but hereditary fashion. It means that they don't own the land, only administer it, but that their descendent can inherit their role.
They vary in size from large cities to fly speck villages. However, none can rival with the large cities of the heartlands.