One of the bigger surprises in gaming last year was 2K Games' XCOM: Enemy Unknown, Firaxis Games' latest take on the classic strategy/action franchise. It was a bit of a gamble – the game was developed to replace the troubled XCOM project that essentially became The Bureau – but it paid off, selling quite well and earning critical accolades.

Now, Firaxis returns with a new story that ties in with the Enemy Unknown legacy, and offers a much more challenging scenario this time around. In Enemy Within, the aliens have launched a fresh assault on Earth – and you'll also have to contend with a new enemy force.

Most of the gameplay tactics from Enemy Unknown are intact here, as you'll once again command soldiers to reach locations on a map and let loose with attacks on adversaries. That said, there are some slight changes to formula, particularly when it comes to challenge. 

Aliens are a lot nastier, with two new classes joining their ranks. Perhaps the most dangerous is the Mechtoid, a heavily armored unit that takes quite a bit of damage to bring down. Once you do, you'll reap the rewards by collecting MELD, a new resource that allows you to open up research and upgrades to your current weapon supply.

As for that other force you'll have to deal with in the game, that would be the Exalt. These mercenaries are people just like XCOM's agents, but they operate under their own agenda. While they aren't too fond of aliens, they aren't siding with them, instead opting to stay out of the way and complete their own set of missions. 

They do still pose a threat to XCOM, though. They can use hacking abilities that can deplete your agency's funds at a moment's notice, and also mess up research if they get deep enough into the system. What's worse, Exalt has made it a life mission to discredit XCOM by any means necessary, not only halting progress through the game, but also turning people – and countries that pay out the nose for alien-killing services – against them.

As a result, you'll have to deal with Exalt in battle just as often as you do the aliens – and these guys don't fight fair. They use the same amount of technical equipment as XCOM, including rocket launchers, grenades, and any tech they've managed to develop from stolen data. They're smart opponents, as they'll use battle tactics to stay out of firing range as often as they can, while at the same time launching special attacks on your teams. For instance, if your squad is gathered close, you should probably anticipate an incoming grenade or rocket attack – something that can damage all at you in close proximity. 

The main goal is to find Exalt's base of operations. With each new mission that's completed in the game, you get one step closer to doing so, even if they attempt to sabotage your search in the process. This, on top of stopping the alien horde, gives you plenty of objectives to complete in the game.

Like Enemy Unknown, the stakes are high in Within – maybe even higher than ever. If you lose a soldier on the battlefield, they continue to be gone for good, unless you somehow reset the game and go back in with your squad before it saves. You can always develop new resources and team members at your home base, and perhaps make even better ones with the new Gene Mods.

Utilizing these, you can physically enhance your operatives' abilities, depending on your resources. These include augmentations to certain body parts, such as chest, brain, eyes, skin and legs. Want them to move faster? Stick to the lower half. Want better battle tactics? Make them smarter. The choice is yours how you wish to modify them. 

In addition to the classes from the previous game, Enemy Within also includes the MEC, short for Mechanized Exoskeletal Cybersuit. This gives you impressive firepower in battle, with prioritized focus on weapons and defense. You can also fit it with better weaponry, including a flame thrower, which does quite a bit of damage to human and alien alike. How do you like your ribs?

Along with the single player campaign, Enemy Within will also include all new tactical challenges to compete across over 40 maps, as well as enhanced multiplayer action, with additional maps, units and abilities available. Again, most of the gameplay remains the same, but if you've got friends to battle against, you won't want to miss out on this action.

While XCOM: Enemy Within isn't a full-fledged sequel, it is a welcome addition to the action/strategy universe, especially with its new additions, maps and enemies. Fans can check it out when it releases on November 12 for Xbox 360, PlayStation 3 and PC.