This is really close to a great game, but it misses on some key ideas. There's no way to grind for money, there's no extra reward for increasing your score. Basically, there's no replayability. A lot of the units seem worthless, and because of the aforementioned issues, they can't be explored.
The unit choosing and upgrades and spells are all fantastic gameplay elements, however. Good job on that.
As I see it, the following gameplay issues can be easily fixed. Hotkey for boost. Powerups need to be more plentiful or a warning system put in place. Mutated zombies need way more health.
How do you scroll the map to see the top zones? There's like 50 comments in here from people having the same problem. Guaranteed + votes for an answer!
On 20, start with 5 towers (cut the trees), then bottom lighthouse, top boat, top lighthouse. You can leave the bottom boat, but keep some mana in reserve at all times. Be ready to use spells on the divers and heroes.