I got this game a while ago and forgot about it. I dont recall whether it was free or not, but having played about 20 hours now I can say for sure itd be worth paying for.
The game is brilliantly designed and well balanced. Its optimization is also perfect - my phone doesnt even get warm when playing hours on end.
The games difficulty is also perfect. I often had to replay a level to beat it on normal difficulty, but not all enough to make it tedious. Each level can be beaten with skill alone - in app purchases are not needed, nor solicited - I hardly knew they existed.
I highly recommend this game to anyone looking for a challenging, and very fun, tower defense game.
MBChBStudent about Kingdom Rush Frontiers TD, v1.9.2