From Wiki Grepolis
Your village's resources are stored in the Warehouse. The higher the level of the Warehouse, the more resources you may store.
|Level 1 - 9||Level 10 - 19||Level 20 - 29||Level 30 +|
The Warehouse in your village may store a certain amount of resources. These resources can be recalled for use in buildings, units and more whenever you wish. The higher the level of the Warehouse, the more resources you may keep at one time.
Each Warehouse level allows you to store a certain amount of each resource. This can be found out by clicking on the Warehouse or by scrolling over the resources at the top. Each Warehouse also has a hidden chamber, where if you get attacked, a certain amount of resources you have isn't loot-able by the enemy player. The higher the level of your Warehouse, the more resources are hidden and "unstealable".
Certain researches (Ceramics) improve the amount of resources you can store. When you've got full resources and your Warehouse cannot store anymore of a particular resource, the number of that resource you have turns red. A quick way of finding out how much Warehouse space you have left for a certain resource is by looking at the C surrounding the resource.
The Warehouse is available from the beginning of the game. When Colonising, the Warehouse starts at level 10.
The maximum level of the Warehouse is 30. The Warehouse's appearance changes depending on the level of the Warehouse.