Brilliant
Very mellow, a refreshing change of pace from normal tower defence games, and the art style gave it a uniqueness that i find lacking in most other entries on here.
The only flaw i can find with this (not so much a flaw as something you perhaps didn't intend) if you place a pair of fully upgraded tornado towers so that their slowing fields overlap, you can effectively immobilise many of the slower enemies completely.
Perhaps you could disable the 'stacking' of multiple slowing effects from different sources on a single target.