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What are resource items? Tutorial didn't mention anything about port maintenance. Help!

That´s really simple. When you are in the screen where you can upgrade a building you can see the cost in gold and the time it takes to build and the new upkeep to pay after the upgrade. And there is a symbol of the item that is needed to be able to upgrade - e.g. 40 tons of Iron Wares that are used up in upgrading the building.