There are some new towers you can unlock once you get into the fourth tier that have upgrades to let you focus on specific abilities. The Archer Tower, for example, lets you upgrade to a Crossbow Fort. From there you can choose from two abilities, including one that increases range and another that shoots a barrage of powerful arrows. Each game you start fresh at the lowest tier, so upgrading the correct towers to the highest level is part of a winning strategy.
Like many games these days, you also can buy more gems with real money, but I found I can do just fine without ponying up any extra cash. Even when a level seems like it can't be beat, you usually only need to rethink your strategy, so think twice before deciding to spend any real cash.
Update: Attack from the sea
The latest update brings three new stages with a coastal invasion by an army of sea devils. You'll have to defend against six new enemies and endure a boss fight from Leviathan, the god of the deep sea. Fortunately, you also get two new heroes to choose from to help you in the fight.
It's true that Kingdom Rush Frontiers has mostly the same features and gameplay found in the first game, but the gameplay is so tight in this series that just playing on new maps with new monsters and a couple new features is still endlessly fun. If you were a fan of the original or want to play possibly the best game from the tower defense genre on iOS, Kingdom Rush Frontiers is the game to buy. And if Ironhide Game Studio keeps rolling out updates like the latest one, there's even more reason to keep coming back for more.