Currency

There are several types of currency in the game.

Coins
Simplest are Coins, which can be spent in a variety of ways.

Health and Armor pickups can be purchased at Huginn's Shop.

Weapons can be purchased at the Blacksmith.

Offerings of coins can be made to Angel Statues, which have a random chance of paying-out an Upgrade when given coins (generally about 5 coins). The statue will disintegrate when paying out, and is unable to be reused.

Coins can be gambled at Gambling Pools. These take the shape of a large bowl next to a small metal box. Pickups may appear in the bowl when coins are fed to the machine. There is a chance of a coin getting stuck in the machine, rendering it unusable. The pickups that can appear are Health Vials, Armor Suits, Coins, Keys. The machine may also spawn low-tier enemies such as Bats and Engorged Bats, so beware.

Keys
The second currency is Keys, which are harder to find and more valuable. They are used to open golden chests and skull chests, which contain upgrades.

Skull Chests appear in groups of three, and only one chest contains something good. The other chests contain one small enemy each. As the loading-screen tip advises, "gambling is only for the rich."

They can also be used to unlock The Library, which will contain an Ultimate Upgrade.

Finally, Keys can be sacrificed at Devil Statues, which consume keys one at a time, similar to the Angel Statue. The payout is much greater at these statues, although the cost is dramatically higher.