When you battle and prestige, your heroes and your villains will be able to earn more skulls and more hearts at the same time, but your heroes will most likely be ahead of your villains because it takes no time at all for your villains to kill enough heroes to be ready to switch armies. Stay on the villain route for longer than you have to, and do your best to bring them up to an equal number of steps with the heroes.
After you prestige, the castle is where you can buy more stuff with the skulls and the hearts. Each castle only contains one store – either the heroes or villains store. Once you switch armies again, though, you will be able to go back to the other side’s store and buy upgrades for them, too.
The hermit shop allows you to buy all kinds of cheats for 100 gems at a time. All of them are functionally useless, though. All that they do is change the look of your characters. So if you want to change their looks, buy these cheats; otherwise, leave them alone and save the gems for better uses.
When you are using the villains, go to the temple and you will be able to research various boosts that apply specifically to the villains only, such as sheepifying your rivals. Also, hit the pits and you will be able to immediately collect from a gold mine and a gem mine, which give you free gold and gems every 12 hours. Tab over to upgrade your prisoners, as well as to buy a random upgrade that earns a 25% higher gold earning from enemies.
The coin+ button and the flaming shield button will allow you to watch an advertisement once every four hours for big boosts. Hit the coin button to gain a 50% gold boost for four hours. Hit the flaming shield button to get a 50% damage boost for four hours in a row.
When you are on the hero/villain upgrade screen and you see them flashing with an up-arrow next to them, that means that they can be upgraded to the next rank. Tap them and upgrade them, and the upgrade will last forever, including even after you battle and prestige your heroes. You only have to buy the rank once, and then never again.