expanding storage

what is the rule on storage expansion from the castle? why does the cap seem to be at 3? how do i upgrade to a 4 to bring out a slow creature?

so i have rearranged my starting hand just to test out the theories of storage expansion, besides certain buildings and events… any help?

Things that cost four of a resource require a storage building. That hut one is usually best, though sometimes you might want to go with an advanced resource building that provides you one storage as well as generates resources. For gold you might want to go with the chest or hollow log since gold is the most valuable resource.

You could also go with the warehouse, it is just like the castle, but it has the advantage of freeing up the castle to draw cards.