If a sequel basically only adds a few different heroes and locales on top of the original game, is it worth playing? In rare circumstances, yes. And in the case of Kingdom Rush Frontiers, it’s a rare circumstance.

Kingdom Rush was one of my favorite tower defense games on iOS even though I never finished it. When I got a chance to play its sequel, Frontiers, I was determined to finish it, even if I had to “grind” away at many early levels to make enough in-game money or level up enough abilities to tackle harder levels.

If you haven’t played a Kingdom Rush game before, basically you have four different towers that you place in pre-selected areas on a world map to try and stop enemies from reaching your one or two different bases. Each tower comes with its own pluses and minuses.

For instance, archer towers are great for knocking down flying enemies, whereas artillery towers are good for taking out large groups of enemies if you’re not worried about them running past them. Mage towers can only take out certain enemies and knight twoers are great for slowing down your enemy, but often get overwhelmed easily.

Those four main towers return in the sequel, as does the upgrade system for increasing the abilities of each tower throughout the level. You upgrade them with money you earn as you kill enemies.

However, the upgrade system has been altered to make the game more interesting. For instance, Mage towers can be upgraded to Necromancer towers which raise the dead to fight for you.

Along those lines, your enemies are also more sophisticated. One of the most dangerous: alien facehuggers that, if not killed before they reach one of your knights, turn your defenders into extremely strong alien beasts that must be defeated.

Game designers also added in the ability to purchase one-off items with gems you earn throughout the level. For instance, buy coins that give you more money to spend on upgrades/towers during a level or even a nuclear warhead that will wipe off every bad guy currently on the screen. (I bought two of those for the final level.) You can “cheat” and purchase more gems through micro-transactions, but I found it quite easy to earn them through regular play.

If you haven’t played a Kingdom Rush game before and are a fan of tower defense games, pick up Frontiers—it’s easily one of the best tower defense games out on the market. (Until Plants vs. Zombies 2 arrives of course.) And if you’re a fan of the original, you’ll likely enjoy the sequel as much as I did.

Kingdom Rush Frontiers is available now for $2.99 on the iPhone or $4.99 for an HD version on iPad.

* Disclosure: A downloadable code for Kingdom Rush Frontiers was provided to the reviewer by the publisher for the purposes of this review. *