The Storehouse increase your capacity for resources (food, wood, stone, kingdom resources & mead). You should build (and max) a storehouse in each castle and outpost. This is one of the most important buildings in the game, since with a high capacity of food you may hold more troops (delay they "starve time" in case your food usage is negative), have better options to do a "feast" to speed up your troops recruitment and the ability to upgrade other building to high levels (which require more resources). All first 7 levels require wood & stone, as the last one requires tokens.