Guild treasury
As your guild conquers and expands its territory, goods cost rise for your next siege. Your whole guild may need to contribute goods to pay for new army deployment and to unlock defense slots (6 of the 8 slots cost goods), and this is where the guild treasury comes in. When you attempt to place an army, you are first taken to the unlock screen, where you can access - and contribute to - your guild's treasury.
In the image, 5 jewelry are required, and below the requirement you can see that they are already available in your guild treasury. Also needed are iron, cloth, ebony and limestone, which the guild need to collect and pay in order to unlock this slot. Buttons beside the good requirement show what you as a player are able to contribute: in this case, 5 iron, or Diamonds to purchase the goods for your guild.
Only when all required goods are available can the slot be unlocked.
HINT: Granting freedom to a sector your guild no longer wants will also result in goods being paid to your guild's treasury.
This feature is also accessible from your guild screen, in its own tab. To donate goods to your guild, simply select the good from the drop-down menu, add an amount, then hit the donate button: