You could almost guess Castle Doombad is an Adult Swim title without being told. Adult Swim has a very distinct, delightfully diabolical style to its mobile games, and this one is no exception. Castle Doombad contains devious traps, a kidnapped princess, and an evil super villain. However, you are the super villain who kidnapped the princess and stuck her in the dungeon guarded by traps. How's that for a twist?
Castle Doombad is a take on tower defense, but a pretty good one. You have to equip your lair with traps to kill the heroes before they can reach the princess. There are spikes, acid drippers, harpoons, buzz saws, and all sorts of other nasty things. You run your little fortress of death by harvesting the screams of the princess and any souls unfortunate enough to go after her. The bad guys don't believe in solar panels, apparently.
This game costs $2.99, but there are in-app purchases. Yeah, that's a bummer, but it's not constantly hounding you for money. You earn a lot of in-game currency from playing. If for some reason you want more, you can buy it.