Lake or wall contours and forts - You can use a fort to block access to a territory surrounded by lakes or walls. Lakes or walls must form a continuous contour of map cells connected, either by vertical of horizontal sides. No contours without forts are allowed (you cannot block someone just by lakes). You also need to have at least one of your towns to be placed inside contour. After fort is built, only campaigns of fort owner, his/her vassals, allies and order members are allowed to reach cities inside contour. Lakes or walls forming a contour cannot be cleared until the fort is destroyed. If someone captures a fort belonging to another player, the fort will auto demolish in 4 hours, meanwhile the new fort owner will be the owner of the contour for a given period.

To create a fort, create a Mark Tower in the cell where you want your fort to be built. Add the lakes or walls around the area you want to protect. After 24 hours, when the lakes/walls are built, you will have the option to build a fort inside the Mark Tower by clicking the the building.