Thanks for playing!
Spells are not something that you cast yourself. They occur on a cycle and deal damage to your goblins that are attacking the castle. There are also ways to benefit from being hit by certain spells if you get legendary goblin upgrades that matches the spell elements.
First 10 rounds are intentionally very easy especially if you get an early legendary, scaling after that becomes exponential and is much harder after that point.
I cant unhear three days grace now lol