Ghacks reports the latest 1.5 upgrade of DTD.

This is the best thing to happen to Desktop Tower Defense since its initial release. There are now Dark and Morph creeps along with Ink, Snap and Boost Towers. Of all the latest towers and upgrades I would say, by far, the Boost Tower is the most significant. In order to get the full effect of the tower you need to make 8 towers touch it. This forces everyone to rethink their maze strategy in addition to their tower mix.