Kingdom Rush: Frontiers is a sequel to one of my favorite tower defense games for the iPhone/iPad. It follows the premise of the original. Bad guys spawn, you have limited places to put 4 different types of towers, you must kill the bad guys with your towers before they reach the end.
Each kill grants you gold you can use to build more towers. The towers you can use have their own strengths a weaknesses. The arrow tower hits fast but does light damage. The magic tower hits harder but is a little slower. The bomb tower hits hard and has splash damage, but can only hit ground units and shoots very slow. The guard tower spawns 3 little soldiers you an use to block the path and hold enemies in firing range of your other towers. The guards respawn when killed and can't attack air units. Each tower upgrades to a more powerful version and after lvl 3 can split into 1 of 2 unique upgrades which have varying degrees of powerful abilities. After completing a map you gain stars based on your performance. The stars can be used to upgrade your towers in between maps. Each tower has its own talent tree of sorts that grants improved abilities to your towers. You can respec at any time and it is often encouraged to help get past a particular map or challenge.
What is new in this release is the addition of a hero. A controllable unit that has their own special abilities and can level up by hitting/killing enemies. Each level provides points towards the hero's abilities. The hero adds an extra layer of strategy and defense to each map. There are multiple different heroes you can select from prior to each map. Note: some heroes you need to pay extra for.
There are a good selection of maps and varying range of difficulties and challenges to partake in. It's a strategy game that has provided me tons value in entertainment for the small price. Anyone with an iPhone/iPad I highly suggest this game.
The sequel is only available right now on iPhone/iPad but the original is available F2P here: http://armorgames.com/play/12141/kingdom-rush and you can also purchase the game on steam!