Coins are the unit of currency in Kingdom, with which the Monarch can purchase, build, upgrade, and acquire many things.
Coins are stored in the pouch.
There are many ways to obtain coins. The most fundamental and continuous ways to make money is through farming and hunting. Meanwhile, logging and searching for coin chests in ruins in the wilderness are noteworthy one-time methods to get coins.
Most actions in the game require coins to do. Coins are used to recruit vagrants, purchase tools and weapons, build & upgrade various structures around the Kingdom, and to order various tasks be done (such as logging or ordering an assault on a portal). In New Lands and Two Crowns, coins are needed to repair the boat and leave the island.
Other than for spending, coins can also be used as a buffer against direct attacks from enemies which would otherwise cause the Monarch to lose the Crown, the subjects to lose their weapons and tools, and the villagers to go back to being a vagrant.
All subjects (except knights) and the Monarch will lose one coin for each point of damage they receive from the Greed. For example, a greedling can apply one point of damage each time they bump against someone, making them lose one coin per hit. A punch from a breeder however would make a monarch lose more than twenty coins per hit.
The coins lost from a damage received can drop on the ground or in the river. The coins dropped in the water are lost for good. Those dropped on the ground can be stolen by a greedling or recovered by the Monarch or the subjects.
Each type of subject can hold a different amount of extra coins.
Furthermore, coins can be dropped behind fleeing townsfolk to protect them from attacking greedlings — they prefer whatever is closer: stealing dropped objects (coins, weapons, tools or the Crown) or attacking people. In many situations, especially against greedlings, if the direct attack cannot be avoided (by a wall, for example), it's better to drop coins before being attacked, because coins dropped from a physical attack may fall in the river and not satisfy the attacker, who will keep charging.
Coins left on the ground despawn with time. They also tend to disappear because of greedlings stealing them or some subjects picking them to deliver them to the Monarch when the mount walks by them.
These are the achievements directly related to coin:
Only dropping a first coin on Day VIII grants: