The sequel brings new enemies into the mix, plus new towers and a completely overhauled user interface. You've got spells to cast that affect all enemies, and there are six different game modes to play with that range from classic tower defense to capture the flag and maze defense. The first one was a fun desktop variant on tower defense, and the sequel looks like it adds quite a few new gameplay and graphic elements into the mix.
The app is US$19.95 for either platform, and it's available DRM-free from Freeverse's site. Even though we have Steam for Mac and more options than ever for Mac gaming nowadays, it's good to see the distinctly Mac developers still releasing new titles.