The goal is to protect the church and the city against waves of Demon Tricks who are trying to invade the church. The enemies will become tougher as time progresses, but you will earn points/gold during the game so you can unlock new weapons and also make your character more powerful by completing these mini-missions (collecting GJCHS50 codes, revealing Demon Tricks disguised as humans, etc..) and fighting Demon Tricks trying to destroy the city.
You earn points, upgrade your character and weapons, and if youre low on health you can return to the church and heal in the pool of Baptism. As you become stronger you can eventually fight your way to the portal (?) where the Demon Tricks are coming from, and destroy it. The city is saved and you win the game.