Just so you know, anyone who here who hasn't posted Kingdom Rush incredibly wrong.
That is the best mobile game on smartphones (IMO), and by far the best tower defense game. In fact it's a step up because of the DOTA like Hero system in the latest entry's.There's another game that game out last year that's also essentially a clone of the Kingdom Rush Series. It's called Realm Defense, but it's actually really good still. The Kingdom Rush series and Realm Defense average a 4.8 rating on the Google Play Store. This isn't bullshit - they're a legitimately some of the high quality mobiles games you can play through.
Realm Defense follows the freemium model, unlike Kingdom Rush, but it works out because you really don't need to spend anything. You just play through the first two world's, three starring all levels and put it all towards the dragon or Ice Wizard hero as they are the best in the game. I've probably put 10+ hours in this game over the past 3 days and I'm on to the 2nd world.
Other games like Limbo, Momument Valley, Sky Force Reloaded, and a bunch of the Final Fantasy games are brilliant on smartphones, but I'd say the Kingdom Rush and Realm Defense and the closets to perfection.
In terms of replay value or total gameplay time, Kingdom Rush, Realm Defense, Sky Force Reloaded, and Final Fantasy Titles give you a ton of value.
The first
Kindgom Rush is free now. Play that and if you like, buy the other ones.
Realm Defense is free too (as I've mentioned).
Do Google Play Opinion Rewards to earn credit store credits for doing occasional surveys. I did this to pay for all my apps/games.