This game played a fairly big part of my younger years, more than games other kids played like Call of Duty, Battlefield, Terraria, Minecraft, all of those didn't compare to the amount of hours I sunk into this game after school. The story was alright at best, and helped transition through the line of maps, but the gameplay was extremely engaging throughout the setup time.
This game has the most enjoyable tower defense I have played, despite my distaste for the genre. If you ever have a chance, buy the game on steam, (get the DLC, and get some friends to as well,) go to the betas and select REDUX, it's a more updated version that still gets somewhat supported, despite the games age.
Overall, a great experience to grab some friends and kick back for an afternoon with.