This game does not require you to use money. You'll only need to spend money if you have no patience. I'm more or less a casual player and I've done fairly well. You can collect souls daily through your inbox, watching a few ads, doing daily missions, and completing tower levels. Stones and items are best farmed on boss levels. Power stones are the hardest to get and seem to only come from the dark dungeon levels. The arena is where you earn keys, ignore the cheats you encounter and just get the keys. You can earn up to 3 per day, 1 seems to be a random drop.