An idea for a new premium upgrade could be, "Freeze!". It's where when you are about to place a new tower, all the balloons and blimps will freeze and the towers stop shooting, so instead of having to rush to put your spikes down so the balloons don't get past.
Very good game, although I think it would be nice if you could select to shoot something other then the closest enemy. Like the option to shoot the ranged, melee, weakest, or strongest enemy visible to you.
Not really, when I take the crew out of an enemy until they don't have anybody left, they have 0 action points even tho their sails are fully upgraded. If I am wrong here please correct me.
 "Checkers"){kind=icon}
 "Symbiosis"){kind=icon}
 "Bloons TD 5"){kind=icon}
 "Desktop Tower Defense 1.5"){kind=icon}
 "Frantic Frigates"){kind=icon}
 "The Last Stand 2"){kind=icon}
 "Caribbean Admiral"){kind=icon}
 "Knightmare Tower"){kind=icon}