Thanks for playing!
To answer your questions:
When the tower gets attacked your character gets adrenaline rush, buffing their ability. Wizard sprays fireball, witch has no cooldown on her frogbombs, and succubus has way more hearts and charm chance.
Wizard is mainly single target DPS ,whilst witch is meant as crowd control by turning enemies into explosive frogs. Succubus is support by charming enemies into attacking each other.
You can upload a custom map with a high tower hp from the main menu to test stuff out, I posted an easy mode map in the discord <3