XCOM: Enemy Unknown succeeded in reviving a seemingly ancient tactical video game franchise and delivered one of the best strategy gaming experiences of last year. Developer Firaxis Games followed up with more digital content to bolster the experience of the core game, but ultimately failed to expand upon their newfound hit in a meaningful way.

With the release of the expansion pack Enemy Within, the studio makes up for any prior mistakes and creates a drastically improved addition to a game which already kept fans happy.Firaxis promised that the new additions implemented into the core XCOM: Enemy Unknown game would prove to be profound, and the studio delivered on this consistently throughout the entire game. New factors like genetic modification, new enemies, improved AI and much more content flesh out a jam-packed campaign full of optional quests, bolder and unique missions and a much tougher last stand for humanity.

The introduction of Meld - an alien substance used to genetically enhance your soldiers or adapt them to costly mechanized heavy infantry bodies - proved to be the largest game-changer of all. In order to collect Meld, players are given a time limit to find two Meld Canisters located on each Enemy Within battleground they visit. Introducing this time limit forces players to leave the slow-and-steady gameplay mantra most are accustomed to and engage with progressively riskier strategies in order to get the bonus supplies. It's a simple addition to the game, but one which alters the very flow of combat and strategies throughout the entire XCOM experience - and it pays off in dividends.

Once gamers have the Meld back at home base, the genetic enhancements they've research can be placed onto soldiers. MEC soldiers will have their limbs amputated so they can utilize their larger mechanical suits, which leaves a surprisingly amount of guilt for any players who develop emotional attachments to their soldiers - as always, naming them after friends is a common but tragic idea. Modifying a soldier's genes to give them things like two hearts, brains that hurt enemies who psi-attack them and the ability to heal mid-combat impact the way combat plays out for the campaign. Medals have also been introduced, granting permanent stat boosts to those who are awarded them. If those soldiers die, their medals go away with them, adding even more emotional attachment to a game which already has a capable ability to tug at the heart strings.

Enemy Unknown suffered from a lack of variety in terms of its map options. Through a single playthrough, it was common for players to quickly start cycling through maps they'd already played before, limiting the challenge as players figured out the best spots and strategies for each location. Firaxis remedied this in a big way, adding unique maps in abundance and detailing them in ways which make them all feel appropriately individual. Through our playthrough of the entire campaign, not a single map was used twice - and some of the mission-specific maps, like the infamous Newfoundland terror mission, proved to be well-designed and utterly unique from any of the regular map rotation.

Though the game introduces a string of new enemies, like the Matrix-esque Stalkers to the sectoid-driven Mectopod, one of the most meaningful additions is the new human enemy faction. Calling themselves EXALT, the white-collar looking group routinely sabotage XCOM operations and provide missions with an entirely different type of combat. Snipers and rocket launcher-clad heavies are an entirely new breed of foe, demanding new strategies to minimize the potential risk to your squad. Players will also send XCOM operatives on covert operations, having to extract them in unique missions later on. Each covert mission reveals a clue to where the EXALT base is, and eventually the player can deliver a final strike on their headquarters in a satisfying bloody skirmish where most of the strategies environment is easily destructible. EXALT adds a great tactical flare to the game and Firaxis has done a great job implementing the human AI in combat.

Enemy Within certainly throws more enemies at the player than Enemy Unknown ever did, likely to make up for the new genetic advantages the player has. While this delivers plenty of new challenge, sometimes the intelligence of where the enemies drop in from the sky just isn't logical. EXALT soldiers dropping in on a tile between two overwatched XCOM agents, or sometimes glitching out and dropping onto the exact same tile as a soldier, are a minor setback in a great game mechanic. As players progress through the newly added technologies in the late game stage, the game needs to throw more enemies to keep it challenging, which inadvertently creates a hectic and entertaining performance that may surprise even veterans of the series.

At its core, Enemy Within is a fantastic reminder of what an expansion pack is supposed to be. Calling something like this just a downloadable content package would seem like an injustice, as Enemy Within changes the core fundamentals of an already entertaining game and literally improves on all aspects of the experience. For a game which already hinges its entertainment on balancing the gameplay, Firaxis has done a great job in making numerous changes but keeping that edge-of-the-seat philosophy the same. The gameplay is fresh, the content is numerous and the new challenges should entice both veterans of Enemy Unknown and rookie strategy gamers as well.

XCOM: Enemy Within is available now for PC, Xbox 360 and PS3.

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Follow John Jacques on Twitter @Makelevi.