The objectives of the 4X genre -- eXplore, eXpand, eXploit, eXterminate -- make for deep, complex, and challenging games, which is one of the reasons why they're so popular. Most of these games are limited to powerful gaming PCs, but the iOS 4X gaming world has been expanding over the past few years, and there are now a number of great strategic games you can play on your iPhone or iPad.

Some of these games are ports of board or PC games, and some of them have been designed specifically for iOS. All of them, however, will require that you use a lot of strategic thought. So grab a couple of them and have fun!

Starbase Orion ($7.99)

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If you're already a fan of iOS 4X games, you probably know about Starbase Orion, as it's one of the highest-rated games in the genre on the App Store. Taking inspiration from the board game Master of Orion, Starbase channels the tabletop experience into a great mobile app that's a bit dated, but still really good.

You'll choose between five different races, or create your own, and take your race from the dawn of their space age through a galactic conquest that includes exploration, diplomacy, espionage, and space combat. A very strong A.I. as well as asynchronous multiplayer support give you plenty of play options, and with new features in every free update, you can be confident that the game will just keep getting better.

Eclipse: New Dawn for the Galaxy ($6.99)

Another space-based 4X game that's also based on a tabletop version, Eclipse is known for being very deep, with seven different races, a complex technology tree, resource management, and ship customization options. The iOS version of the game is built to be touchscreen-friendly, and is simplified just enough to make it a good mobile experience.

Despite being a very deep and complex game, the developers have scaled it down to a small-screen version very nicely, with the tech trees and upgrade panels tucked away to the side, and an in-game tutorial makes it easy to pick up and start playing without reading through a large rulebook. It's one of the most popular iOS 4X games for a reason.

Palm Kingdoms 2 Deluxe ($4.99)

If you're a fan of the old Heroes of Might and Magic games, you'll find a lot to love about Palm Kingdoms. Originally designed as a Heroes clone, the game has taken on some life of its own and has become one of the better turn-based strategy games available for the iPad. Seven fantasy factions, 180 creature types, and 114 artifacts mean there's plenty to explore and discover, and a wide variety of maps and scenarios to take on keeps things interesting.

The biggest count against Palm Kingdoms is that there's no online multiplayer; you'll have to either take on the A.I. or use pass-and-play with someone that's with you. There are also a lot of maps that aren't available when you purchase the game, and need to be unlocked via in-app purchase (the all-maps mega pack is $8.99). Despite that, Palm Kingdoms is a well-liked game, and will appeal to fantasy 4X fans.

Star Traders 4X Empire Elite ($4.99)

If you're looking for depth in a 4X game, Star Traders 4X is the way to go. With a massive tech tree (almost 300 pieces of tech to research); the potential for an absolutely huge universe (up to 150x150); diplomatic, economic, and espionage options; internal political struggles; and serious planet management, you'll quickly put in a lot of hours without realizing it.

To give you an idea of just how detailed this game is, you can see the population, number of upgrades, number of factories, the current mining status, and morale for every planet. Ships can be customized to your preferences, and you'll need to use them to face a wide variety of challenges. For $5, this is one of the best deals that you can get if you're looking to get into the iOS 4X genre.

Alien Tribe 2 ($4.99)

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By combining the generally turn-based 4X genre with real-time strategy (RTS) and even a bit of tower defense, Alien Tribe 2 carves a unique niche out of the mobile strategy space. As you'd expect, you'll need to explore new planets, harvest resources, research new technologies, build up your military force to protect your refugee race, known as the Fifo, and find the pieces of their destroyed Ark ship.

Unfortunately, there's also an in-game currency called GEOs that will try to tempt you with in-app purchases. You can certainly play the game successfully without them, but the blatant cash grab will likely turn many players off of this game. Still, the cross-genre gameplay, engrossing goals and missions, and nice-looking 2D graphics will help a lot of people overlook this minor drawback.

Autumn Dynasty Warlords ($6.99)

Another game that combines 4X and RTS, Autumn Dynasty Warlords is the sequel to Autumn Dynasty, a highly regarded real-time strategy mobile game from 2012. Warlords sees you facing off against other would-be emperors in an attempt to unify a fractured empire. Select troopers, issue decrees, send officers on diplomacy and espionage missions, and manage real-time battles to ensure your victory over other aspirants to the throne.

Warlords is meant to be very mobile-friendly, and a number of the things brought over from Autumn Dynasty have been simplified to keep the game from getting too complex. With frequent checkpoints and fewer options, it makes for a good introduction to the genre on iOS. That said, the non-linear campaign, variety of missions, and randomly-generated maps will keep you coming back.

Super Tribes (free)

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Unlike most 4X games, Super Tribes is meant to be played quickly. It's a relatively simple game, despite having all of the four Xes, and has clearly been built to appeal to mobile gamers. The graphics, interface, and gameplay are all simple, but actually doing well in the game against the A.I. (as there's currently no multiplayer option) can be surprisingly hard.

One of the most interesting design choices that went into Super Tribes is the absence of a dedicated tutorial; you just start up the game and start tapping on things. You'll get tips as you go along, but for the most part, you'll just be exploring the interface and action options. It's a quick game, but you'll be surprised how strategic it can be!

Empire: The Deck-Building Strategy Game ($2.99) [No Longer Available]

The goals of 4X and the mechanics of deck-building aren't an intuitive match, but Empire manages to pull off the combination flawlessly. . . so much so that a number of people declared it the game of the year in 2013. You'll manage your empire through exploration and expansion, but combat is taken care of with the deck of cards that you build throughout the game.

The battle grid is much like Plants vs. Zombies, in that your units do damage to the opponent's base when they reach the end of the line, a process that's expedited by playing power-up cards. If you lose a battle, you'll end up collecting Strife cards, which take up space in your deck and don't help you at all. The deck-building component of the game has been compared to Dominion and Ascension, two great card games available for iOS.

Your Favorite iOS 4X Games

The 4X genre is full of strategy and decision-making, and these games, despite being played on your iPhone or iPad, deliver in full. Some of them are just as complex as a tabletop game, while others have been designed more with the mobile gaming crowd in mind, but all of them will give you a great dose of strategy, no matter where you are.

This is a great place to start, but what did we miss? What's your favorite iOS 4X game? Leave your suggestions in the comments so we can check them out!