Some people may have had issues with the difficulty. Personally I like a game that takes a bit of time to beat. And beat it I did on every difficulty. There were a few levels that had me stumped for a few attempts but I quickly realised that the shop was there for a reason. All I had to to was try out a few different strategies while collecting gems and eventually I'd hit the combination of correct upgrades and correct in-level tower choices and I'd win through.
If you are having trouble I recommend trying different strategies. Try building a lot of cheap towers rather than mixing them or spending the in-level coins to make them stronger. Or try concentrating on one good tower rather than a few weaker ones... whichever tactic you haven't yet tried.
The game is a lot of fun and I beat it on the hard difficulty long before I ran out of available upgrades. I intend going back to it in a few days and trying to collect enough gems to upgrade everything fully, just for the fun of it.