Just played this all the way through... again.
The art, first off, is top-notch. It's not too complicated, but it's right in between cartoon-y, and serious (those spiders still freak me out a little bit). The amount of detail put graphically into each level gives it depth. I like that in my free strategy games. The tower designs are great to look at, and the differences between them shows just how much thought went into everything.
The gameplay's perfect. A few slowdowns, but that might be my computer. The level of difficulty rises gradually, and the extra challenges keep you wanting more! The various sound effects and voices likewise add levels of depth. The achievements more so, although I would've liked them to be linked to Newgrounds medals.
This game demands a sequel. This game NEEDS a sequel. With a delicately crafted world like this, you could spawn several, and keep the interest of fans. I wanted so much more, I paid for the premium content... but honestly, I didn't want it to end! You could make a game three times as long, with quarterly new downloadable content, and I'd play for weeks.
This game is incredible. For a free game, or something you pay for on a tablet. One of the best strategy games on here, bar none.
P.S. Make a sequel.