Fuu-Un! Dairoujou (DS).

I've been playing a couple of stages of this every other night, and I am enjoying it tremendously. It's a somewhat simplistic tower defense game, but a very charming one with a good deal of challenge, as well as an extremely well-measured difficulty curve. And it's only 8 MB! When I consider that crap like ASH comes in 256 MB carts, and that this little game still manages to be prettier than that, more engaging and more challenging, I just wish these nn.system guys had Sakaguchi's budget.

Chances are they'd then make some shitty J or SRPG though...

At any rate, this game comes strongly recommended by me.

Edit: Forgot to say: knowledge of Japanese is not really necessary apart from understanding what's been said in the dialogue scenes of the story mode (which I am not playing -- I am simply taking on the stages in the first "free" mode (of which there are two)).

This is pretty freaking awesome:

http://nnsystem.jp/dairojo/ds/trial/load_game.html

Not really worth playing, since you can only use three kinds of units, and the stages are piss-easy, but this basically runs on a fully functional Flash DS emulator!