The Cranny is used to hide some of your resources in the event the village is attacked. Resources that are stored in the Cranny cannot be stolen by attackers. The amount of resources hidden is dependent on the level of the Crannies that are in a specific village.
After one Cranny has reached level 10, additional Crannies can be built.
Requirements: None

The amount of resources protected depends on the level of the Cranny. If you have multiple Crannies in the same village, their protection capacities are added together.
Crannies are particularly effective for Gauls, as their Crannies protect twice as many resources.
Teuton Heroes have a base plunder bonus of 20% when they accompany an attack. This reduces the effective protection provided by the defender's Crannies.
Bags equipped by an attacking Hero also provide a plunder bonus that reduces the effective protection of the defender's Crannies.
For example, a 10% plunder bonus reduces the effective Cranny protection to 90% of its normal capacity.
If a Teuton Hero accompanies the attack and is equipped with a Bag, the Bag's bonus is added to the Hero's base plunder bonus.
Learn more in Hero Equipment: Left Hand.
Crannies are especially useful if your village is being raided regularly and you cannot yet defend it effectively. Reducing the resources an attacker can steal makes your village less profitable to farm.
Learn more in Help! I'm Getting Farmed!