currency

copper coins are mice. anyone who can carve or stamp a mouse on a piece of copper, that's a valid coin. a live mouse is worth exactly 1 copper. you can trade with live mice if you don't have copper ones. also, you can eat the mice (mice are a staple of the peasant diet).
silver coins are owls. 1 owl is worth 10 mice. owls are made in presses, but each coin maker has their own press, with their own markings.
gold coins are sentinels. an owl-sentinel is a mythical blackbird that hunts at night and eats owls. nobody's ever seen one, but they find casings. the picture on the coin has about ten legs. 1 sentinel is worth 10 owls. by law, all sentinels are made in the royal press at alexandria.
an ounce of salt is worth 100 sentinels.