Resources: What to do when resources run out
If you are unable to collect certain types of resources at the beginning of your turn because the stockpile is empty, you may collect the same number of resources of another type, not including Craftpoints.
Don’t forget, you can always trade for other resources at a 2:1 ratio during crafting, even if there are no more in the stockpile.
Example:
A player wants to recruit an archer, but there are no more wood resources available. That player trades in 4 iron for 2 wood. They continued to purchase the soldier as normal with the rest of the resources and coins they have on hand.
The above example works even without wood resources in the stockpile. It is a direct trade-for-purchase scenario.