Looking for deep, challenging games, with a dose of complexity and strategy? Then you're probably looking for a 4X title, where the objective is to eXplore, eXpand, eXploit, and eXterminate in order to dominate and win the game.

Many 4X titles can be played on desktop computers, but they're less common on mobile. We've previously taken a look at the choice of 4X strategy games for iOS, but what has Android got to offer?

1. Civilization Revolution 2

We'll start with the most popular name in the world of 4X, Civilization. This mobile version is the first time any title from the Civilization franchise has appeared on Android, although the original Civilization Revolution was available on iOS and even Windows Phone in 2012.

Released in 2014, Civilization Revolution 2 brings a slightly simplified version of Civilization V to your Android smartphone or tablet, with the turn-based 4X strategy game challenging you to take command of a famous civilization in human history and lead it to greatness.

The game is won via one of these four strategies: Domination (capture all enemy capitals), build the world bank and collect 20,000 gold pieces (Economic), convert 20 cities (Cultural) and Science (build a spacecraft and launch it to Alpha Centauri). How you achieve victory is up to you, but there is no set path.

Compared to Civilization V, this mobile version is almost a different game. It probably feels closer in depth to Civilization II, but this is no bad thing. What might be a problem is the price: $9.99 could be considered a bit steep for a mobile game, but on the other hand, there are no in-app purchases.

2. Rebuild

Most 4X games are set in the past, or in space. They almost always feature a randomly attacking enemy horde, such as the Barbarians in Civilization.

Rebuild is different.

Rather than placing you at the head of a tribe in the distant past or a group of colonists in the present, Rebuild drops you straight into a whole lot of trouble -- think Dawn of the Dead (or Shaun..., if you prefer!) and cross that with the typical elements of a 4X game and you're pretty much there.

There's no need to build your empire in the usual way, as your main aims are to gather the survivors of a zombie apocalypse, defend your fort, and expand your defensive unit by recapturing one building at a time. Killing zombies is a priority, as is rediscovering technology, but you'll have to be on your guard as rival bands of survivors can prove challenging, as well as disease and crime.

Available for just $2.99, Rebuild is an entertaining spin on the 4X genre. It doesn't try to copy Civilization, and in a genre full of Civ clones, that is a very good thing indeed.

3. Star Traders 4X

This RTS game is a strategy take on the Star Traders RPG, which is set in the same universe; and challenges you to protect your people, manipulate rivals, unlock technology and build a galactic civilization. If you can manage the economy, realm, and scientific development, you have a chance of success, but only using espionage and diplomacy to gain an upper hand.

A useful tutorial helps you to get started, although you can easily skip it.

Like free 4X titles? Check out Star Traders 4X and get lost in it!

4. Imperium Galactica 2 [No Longer Available]

Available for $5, this 4X RTS is a classic game brought back to life on Android.

Three playable campaigns are available for you to build massive, universe-spanning empires, with the option to use diplomacy or military force to dominate your enemies.

Renowned as a genre-defining 4X game, Imperium Galactica 2 features space and land battles, economy and population management, and customizable military units, among many other features. Forget about this being a paid title and give it a few hours of your time!

If you want more like this, check out our roundup of the best war strategy games.

5. Freeciv

This one has been around for a while, and is an open source clone of Civilization II, the 1995 sequel to the 1991 original. The similarities are stunning, and while Freeciv is perhaps more widely played on the desktop, it has its band of supporters on Android.

It's hardly a surprise. Back in 2014, this was the only Civ-like game for Android, unless you were prepared to run the original in an emulator.

There is one aspect to the game which might put you off, however. Despite the game being available for over five years, the developers have struggled -- or completely ignored the need -- to develop the user interface. The result is a slightly clunky experience that tries too hard to replicate the original desktop version, overlooking the needs of a mobile user.

But if you're prepared to overlook this, you'll get the full Civilization II-style experience, right there on your Android phone or tablet!

6. Populus Romanus

Think yourself as a Roman emperor? Populus Romanus puts you straight into that role, with a choice of 14 scenarios recreating various real-life scenarios, such as the First Punic War. Styled like a tabletop board game with hex-and-counter gameplay, this is an advanced strategy war game -- with a bit of Civilization mixed in.

You're thrown into the action immediately with the built-in scenarios, so you'll need to be quick-witted to work out what is going on, how to move, and how to engage in combat. Once you've got this worked out, however, and your cities are creating more units, the game becomes more strategic.

Populus Romanus is pretty addictive once you get over the slightly odd graphics.

7. MyCiv Alpha

Still in the development stage, MyCiv Alpha has a long way to go but marries polished graphics with Civilization-style gameplay -- as you'll gather from the title!

The game has quirks -- it's not entirely stable, the menu screen is portrait while the game itself plays in landscape mode (and neither will switch when your device is oriented) -- but it also makes use of the touchscreen to add some useful gesture support, such as pinching to zoom in and out of city view.

But there's a problem with pace, and a lack of movable pieces following the first turn means you're sat hitting Enter until your first piece is ready, a problem fixed by various other 4X games long ago.

If you're somehow still looking for your Civilization fix on Android despite Freeciv and Civilization Revolution 2, then MyCiv Alpha is certainly worth keeping an eye on.

8. Birth of the Empires

Think Microprose's Master of Orion 2 or Star Trek: Birth of the Federation, and you've got a good idea where Birth of the Empires is coming from. This turn-based 4X title puts you in charge of one of six different races, including humans, aiming to win by gaining diplomatic or military supremacy over the galaxy.

It will take some time for you to get going with this traditional, static-screened 4X classic, but once you've got started -- as with all of these titles -- you'll find yourself hooked, and falling back on the old "just one more turn" catchphrase at three in the morning.

For additional strategy games as well as other game genres you might enjoy, check out our suggestions.

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