Fortresses on the world map.

Sometimes barbarians can take over your province and build their fortress there. During that time, while this fortress is located in the captured province, this territory will not produce products or pay taxes. It will be very difficult for a single player to destroy the fortress. Use the forces of the entire clan in the most optimal way.
Only one player can attack the fortress at a time. After dealing damage to the fortifications, the player can retreat and allow another clan member to attack this fortress. Each player who attacks the fortress will receive a reward in case of victory.
Or you can just wait for the barbarians to leave by themselves. The time left before the barbarians leave is shown on the icon on the left side of the screen. Attention! If the barbarians leave before you destroy their fortress, no one will get a reward.