Theres a part of the game where you can jump into a well, leading you back to a village. There are bats in this well. Attack and kill them, grabbing what goods they drop, and then jump back out of the well. Jump back in (as the enemies have respawned below), and repeat this process for infinite funds.

To unlock all of the in-game artwork, simply beat the game once through.

To unlock a harder difficulty setting than is initially available to you, beat the game once through.