Once upon a time XCOM: Enemy Within was going to be a simple add-on pack for Enemy Unknown, but it ended up adding so much to the core game that 2K opted to release it as part of a bundle that includes the main game and all the previous DLC.
Enemy Unknown takes place on a modern Earth, under attack from an alien invasion. Facing an unseen enemy of clearly superior technical capabilities, Earth's governments combine to create X-COM. Playing the commander of this global force, your mission is to create an operational base, research alien technologies, organise combat missions, and control your soldiers in battle. You'll dispatch aircraft to intercept and destroy or capture UFOs, and manage ground forces in their battles with alien invaders. All of this is controlled from your XCOM base. Yes, that's base in the singular, unlike the 1994 original, only a single base can be constructed and managed: this adds a strategic element to the game, because your base has to be defended at all costs.
The forces you command are not your own, but like the United Nations, you act a liaison between many disparate governments, and this adds to the tactical aspects of the game.
The Ant Farm view gives you an overview of the XCOM headquarters: you can then zoom in to individual departments to direct specific activities such as research and weapons construction.
Battle strategy takes pace on the Geoscape global view of the world, and you can be directing battle in many theatres of combat simultaneously. Ground combat is turn-based on an isometric-3D playfield. You directly control a squad of four-six soldiers or robots. As you would expect from a modern game, the user interface has been updated considerably from the original game, and gameplay has been tweaked to make it faster and more engaging. Players can make use of suppressive fire and active camouflage during combat.
One-on-one on-line combat missions enable players to control a squad each, and have a points budget to assemble and equip them before leading them into battle against your human opponent.
Console players will be happy to hear that they are not being offered a dumbed down version of the game. Apart from the mouse-based control method, which is exclusive to the PC version, the console owners are getting the same game as PC-gamers.
Enemy Within introduces a host of new abilities, upgrades and weapons as well as maps, strategic and tactical gameplay.