If you like tower defense games this is a must-have title. It reminds me a little of Puzzle Quest or Clash of Heroes in that it adds RPG elements to a strategy/puzzle genre with great results. The game is pretty long and has good replay value, including a "new game +" mode that adds extra missions, new cut-scenes, etc.
It may look simple and retro, but the gameplay is among the best, the music is very good, and the humor is a nice touch. The price may seem a bit steep given the 16-bit graphics, but it's well worth that price.