Can be upgraded up to level 45.
At the start of the game you can only carry five items in the backpack. To be able to store more, you can upgrade the treasury and unlock up to 15 more slots in the backpack and up to 30 more slots in the inventory chest.
Upgrade levels 20+ will be only available after upgrading the main building (fortress) to level 20.
The secondary function of the treasury is the secret storage. Wood and stone found on quests or gained from the wheel of fortune etc. are stored and secured here.
The size of the storage is 30% of that of the mainfortress, and all resources in it are protected from attacks of an enemy.