Coin gain is mostly depending on the units, buildings and ressources produced (killed ?), so longer games are more rewarding. It's actually pretty easy to grind money that way.
Level 5 tip : Towers right side of your castle, teleport spell right under the lake, use fireball to destroy enemy catapults. In fact that level is easy.
Level 10 (actual limit) and boss 7 down. Very nice game, little hard on some fights (specialy on bosses or multiple path fights) it takes few try to learn the bests options.
 "Ancient Planet"){kind=icon}
 "Rise of Mythos"){kind=icon}
 "Stormy Castle"){kind=icon}