If you look at Ninjatown with a magnifying glass, it is easily seen that this game was created using nothing but existing gameplay mechanics from games before it. The key here is that this title has so much unique personality, it really gives the RTS/Tower Defense genre a whole new feeling. For the most part, the simple 2D art design works extremely well in generating the game's overall appeal; it just could have used slightly more definition in terms of upgrading structures and deploying more powerful units.
However, toss in both a single and mulit card link mode, and Ninjatown is really one DS game that should receive some positive attention this holiday season. If you want to try before you buy, there was a downloadable demo available via the Nintendo Channel on the Wii.