The Arrow slit is a tool to greatly help ranged defenders defend in battle. If you have done the Hall of Legends skill "Technical Innovator"", you can have a nearly undefeatable flank or front without using Shield walls.
Example of defenseEdit
100 ranged base %
+90% ranged bonus from castellans
+350% from 5 of this tool=
540%, which means the Veteran Composite Bowman has a ranged defense of 988.2 or with L5 Towers without any bonuses, the ranged defense is 316,224!