played it...
couldn't finish the last level. Damn you for wasting my day! And i haven't discovered what the last combat or magic tower is.
Strategies i've found that worked (Catapult + R rune (homing)) is very good for ground units (Floating and underwater), I usually use 4+ each map (later maps lots! due to lots of cash)....
You need to keep using the Platform ability everytime it becomes available else you can't win some levels, (allows placing more towers, creating bottlenecks ..etc).
Falcons plus H rune (damage increase) are awesome at late levels for bosses and all. I use 5+ at the end. Air is the bigger challenge in the end, have to use the special ability axe on them...