Tower defense games are a subset of the real-time strategy genre that runs rampant in the iOS gaming world. These apps require quick thinking, careful planning, and speedy finger-tapping.

1. Kingdom Rush

Kingdom Rush and its collection of follow-ups exemplify fine-tuned, slick design found in the tower defense genre. Although the medieval aesthetic doesn't stand out from the crowd, this game buries a lot of complexity under the hood, allowing for a diverse range of tactics rather than just one winning strategy. Although in-app purchases do appear in the game, their implementation is kept to a tasteful minimum.

2. Green the Planet

This relatively new game isn't as difficult as some more recognized stalwarts from the genre, but the cute setting makes for a compelling spin: You play as a band of aliens fighting to terraform and protect a planet, fending off meteors and decomposing comets in order to keep your future home (or humanity's future home?) safe from preemptive destruction.

3. iBomber Defense Pacific

This military-themed game—as well as its predecessors and spin-offs—bombards you with attacks from all sides, so be ready to act fast. The game uses real-world locations and weaponry but twists reality enough to come across as light-hearted and fast-paced fun rather than serious historical drama.

4. The Creeps

This tower defense game originally came out in 2010, but unlike other "classic" mobile games, it's still getting updates every few months to keep the fires burning. The Creeps is a must-play for tower defense mobile fans, especially those who may not realize how many other TD games have mimicked its stylings over the years.

5. Prime World: Defenders

In addition to being one of the more visually striking entries on this list, Defenders implements some collectible card game mechanics into its design, demanding that you unravel different types of tactical trickery in your brain in order to understand all of this game's mysteries.

Your favorites?

I intentionally left out some better-known apps like Fieldrunners and Plants vs. Zombies, but this list is also probably missing some lesser-known tower defense titles you might fight to the death for. Which TD games have you loved that didn't make the list?