Oh yeah...while playing I repeatedly got a quote from Cloud Atlas in my mind: "The weak are meat the strong do eat.".
Once you are around lvl 10/11 you (or at least me) will probably get a ton of attacks from players who attack you with planes. Funny thing is that by normal gameplay you cannot have planes for yourself until you are lvl 13 (or better 14-15 as you first probably have to go through the lengthy process of building new energy buildings and an airport).
Well of course .... you CAN buy planes before. All you need is "a little" real money investment to buy some planes (which you will lose no matter what after a couple of fights).
But once you yourself are lvl 14/15 you will probably do the same to the lower levels as you get a task that asks you to increase your "fame" drastically ;-)
The same probably awaits you when you get into the area where underwater warfare gets available - just that it will probably take much longer to get through the "dangerous" levels and make underwater units available to yourself.
For sure, kudos to gameloft - from an economical point of view this is really well developed. Where Dungeon Hunter 4 was mainly annoying with reminders that you should buy something for "real money", this game used every chance to tell you "Don't be meat - be strong! Get the good stuff...".
Oh, how much I "despise" everybody who made this annoying trend in the gaming industry possible....