Defense games?

Are there any Desktop tower defense type games on the psp? I really enjoy Crystal Defender on my 360, and Am looking to be able to play something like that at work on breaks and what not... Thanks in advance :D

Other than maybe putting some flash games on your psp, (I've read thats possible but I've never tried it) I don't know of any games where you defend. Sorry.

PixelJunk Monsters is coming to the PSP this year. Right now, I believe that you can play the PS3 version via remote play.