Well....
I loved the idea of using Land, sea and air but it became boring too quickly.
It would have been more interesting if more towers with different abilities were introduced (fire, ice), the ability to upgrade different stats on the towers (Range, strength e.t.c.) or to have challenges or different difficulty levels.
Was a good game but needs more content.
N.B. Was there much point in having 2 separate upgrade buttons? 1 would have done