This game is best suited to players willing to pay to progress or those with the patience to be in it for the long haul. Its beautifully designed and well thought out, very stable with no crashes. Very enjoyable in the beginning but then, seriously unbalanced from the Bird levels onwards to coerce IAPs. You will be attacked by increasingly overwhelming waves of enemy while it will take much longer to collect the necessary cards to upgrade your towers to field a worthy defense. Theres a huge amount of content but, a lot of it is quantity over quality.
Disappointingly, 75% of card drops after much grinding are worthless. I have dozens of cards Ill never bother upgrading. The arenas and outpost maps are boringly repetitive and you will have to play them hundreds of times (Ive played the various arenas over 1600 times!) to be able to obtain worthwhile cards. Then, it will take at least 3-4 days to upgrade to the higher levels of the more useful cards (which will literally take months to acquire in the first place). And, you can only upgrade 1 card at a time!!
Unless you pay up, of course. Even then, You will need to spend at least $50 on gems if you want to quickly upgrade a couple of rare cards beyond level 5. I bought some gems but, the cost of upgrading cards is so high that I went through 3000 gems in a few minutes. Be patient and enjoy the game for what it is but dont bother buying gems or cards unless you have hundreds of $$ to waste.
Yes, Devs deserve to make money but, their shortsighted greed has ruined what could have been a truly excellent game. Balanced gameplay with the option to pay to progress more quickly would have been a better strategy rather than trying to coerce us into it. I would even have gladly paid upfront for a couple of levels and more periodically for extra levels.