Black Matter = Free.
This is an old (by ios standards) TD-shooter hybrid, IIRC. I used to have it but it was on the Apple account I had to close, so I haven't played it for a long time. Having said that, I do recall enjoying it at the time on my old ipod touch 1 or 2, and I hate TD games. Maybe because it masks the TD style by having no paths...anyway, its worth checking out. It hasn't been updated since 2010, so I'm not sure if its retina, but the filesize is only like 8 megs.