For the original DS there's a port of Desktop Tower Defense, which is pretty unremarkable, but there's also Ninja Town, which is fairly easy, but rather amusing. --- 27 games reviewed to date; latest review: Legend of Zelda: Link's Awakening (GBC), Donkey Kong Country Returns (Wii) http://backloggery.com/94067
There's Ninjatown, Plants vs. Zombies, and Fieldrunners (DSiWare). I've downloaded the last one, but have never actually played it.
Just recently, I beat Plants vs. Zombies on the PC and have been helping a seven-year old with it on the DS. After beating Ninjatown months ago, I'd say it's far better than Plants vs. Zombies. Definitely longer w/ more levels! I would suggest going with Ninjatown over any of the others. --- http://www.backloggery.com/melikupo
Lock's quest for the DS was really fun and interesting, though it is slightly different from your regular TDs. --- "You're in America now, Our idea of diplomacy is showing up with a gun in one hand and a sandwich in the other and asking which you'd prefer."