While far from deep, I'm finding it an amusing time waster.
Each round you fight off a wave of heroes trying to raid your little grid of monsters and traps with you on the other end to pick off any straglers. After you clear the wave you'll then get to make some minor upgrade (like a new trap, a new monster, a new piece of equipment, or to upgrade either yourself, a monster, or trap). Rinse, repeat.
You start the game with 2 Dark Lords to choose from but, with enough play, you'll be able to unlock several more all with their own unique powers to use in future runs.