Do not spend your Skulls! Heroes will sometimes drop this very rare loot, and you can use it to call on some extremely powerful magic. You will not be getting very much of it during the course of the game, and you’re going to need those spells in the last few levels.
If a hero carries one of your gems away from the cave, you can use terror magic to have another bring it most of the way back. Undead towers can use something similar to this magic, so it’s helpful to build at least one of those towers along the main route to the cave.
The terror spell can also be used to remove negative effects from your own towers when annoying heroes hit them with their skills.
The little scrolls dropped by downed heroes now and then will freeze nearby targets when you tap them. Don’t use them until there’s actually something worth stopping around them.
It’s best to distribute your points that you get from leveling up evenly among the three groups. Level up the demon skill Infamy early on to bring in more experience points more quickly. It’s worth a lot in the long run.
Keep an eye on flying heroes to see which shortcuts they’ll take, then build the appropriate defenses to clip their wings.
Take advantage of higher ground. The game will tell you this, but as a reminder, your towers will be able to attack at double the range if you built them on high ground.
Don’t worry about getting three stars on your first trip through each stage. Just focus on moving forward. If you hit a tough stage, head back and try to get more stars on earlier ones. It’s a more fun way to grind out the experience you need.
Capture mines as early as possible. You’ll definitely get your investment back in spades provided you get to them near the start of the round.
Every hero has their own special abilities. Make sure you know what you’re dealing with by checking the Evilopedia often!