Strongholds are heavily fortified locations that can be found in the Outer Empire and Frontier regions. These locations are incredibly well-defended by Barbarians and are notorious for sending Barbarian Raids out into the Empire.
Strongholds are also able to control surrounding Towns and Villages. To free them, the Stronghold must first be Dominated. (Control the Greater Stone Circle in the middle of the camp).
To put an end to the raids the Stronghold needs to be pacified. To pacify a Stronghold you must control all Stone Circles. Each Lesser Stone Circle you control will slightly reduce the size of Raids the Stronghold can produce.