Market barrows are used to transport resources between castles & outposts. The total capacity you can transfer is determined by your "Marketplace" level, build items attacked to it & research. You may also increase the capacity of a single barrow at the "The King's Market". Total capacity in a multiplication of the total number of barrows by its capacity. You may upgrade your Market barrows capacity by paying with rubies, up to 251 per one barrow (total cost to maximize will be 44,000 rubies). Every upgrade you do is permanent.