Last year's XCOM: Enemy Unknown was a critical darling, redefining how a turn-based strategy game can look and play. A year later, Firaxis returns with a new Civilization-style expansion pack dubbed Enemy Within. Featuring the same aliens-are-attacking story as the original, this expansion remixes new missions and enemies into an already-existing framework. It’s about the "how" and not the "what" as Enemy Within leans on substance to make its case.

The core of the game still consists of taking a six-person squad through grid-and-turn-based strategy gauntlets with an emphasis on cover and sight lines. While it may look largely familiar, all the systems under the hood have undergone massive changes. Enemy Within expands upon micro-management significantly, making the original game feel rather shallow in comparison. There have been so many elements added to the game's already-complex design that it can almost be overwhelming.

Most of the tweaks have come to soldier development. Yes, permadeath is still essential to the game's design. However, you're given a lot more flexibility in soldiers are constituted. One of the big new systems is soldier augmentation, where soldiers can be turned into a cyborg capable of wielding one of the game's new mech suits. You can also alter a combatant's genetic code to provide super-human abilities. There are plenty of new weapons and ammo types, making research and alien autopsies even more essential when acquiring these new abilities and technologies. There's even new resources to collect, like meld canisters that appear in each mission.

The problem is that every last one of these new systems requires money to operate, in addition to the other responsibilities you have as commander. You still need to save copious amounts of cash to launch satellites and fighter jets to protect all the countries in the XCOM alliance. You also need money to build facilities to make blueprints a reality. All told, the new features result in a pronounced grind for cash, particularly at the end of each month as you await the scheduled money infusion. You can get to a place where all your high-level soldiers are injured and the only way forward is to replay each mission until you perfect it. Even more than before, the constant threat of worldwide doom makes every battle matter.

There are new foes to deal with as well, include cloaked creatures called seekers and a faction of traitorous human enemies called EXALT. Using weapons similar to your own, these 80s spy movie agents also utilize gene manipulation to alter their abilities.

It will take skilled players well over 30 hours to get through the campaign, and like many strategy games, it provides near-infinite replay value as you try out new augmentations, skill trees, and strategies. The head-to-head multiplayer across 13 maps is fun while trying out all the new toys against an unpredictable opponent. However, some strange camera decisions and a lack of information in the menus make it more of a struggle than it should be. It’s hard to take it too seriously when so much is predicated on luck.

Luck is something that comes into play no matter the mode. Missing a high percentage shot when you need it most can be absolutely maddening. Knowing that your plan can go awry at any moment, an air of uncertainty permeates the entire experience. From the sometimes-confounding fog of war to enemies that just randomly drop from the sky to completely foil a meticulously-laid trap, the game can come off as unjust. Some will say, “that’s XCOM.” And while that’s true, it doesn’t make it any easier to digest.

There are so many variables at play in any given move, that you can take 15 minutes thinking about each one and still immediately regret your decision. Altitude, line of site, destructible cover, movement, weapons, enemy type, buffs and more must all be considered. Knowing one misstep can foil everything imparts each click with equal bursts of anticipation and dread. It’s what makes XCOM so addictive, and Enemy Within spills over with it.

Firaxis hasn't done much to make the original story any more interesting. For example, the introduction of EXALT is largely cosmetic and is one of the expansion's flimsy new plot points. And in spite of the expansion's new maps, the late game nonetheless feels monotonous due to its marked cut-and-paste feel. Maps and environmental elements repeat, new enemy types are introduced at a drip, and the objectives rarely deviate from escort missions and killing everything in sight. EXALT missions have players extracting agents. It’s king of the hill, an escort mission, and hacking rolled into one, but it's not enough to alleviate the repetition.

Visually, Enemy Within isn't drastically different from the original game, utilizing the same engine as its parent. On PC, mid-range cards and rigs can handle it fine on max settings. The action cams return and continue to give XCOM a visual identity most games in the genre lack--there’s nothing quite like a robot punching an alien through a train car.

The PC version requires Enemy Unknown, and includes the previous Slingshot and Second Wave expansions for $30. On consoles, it’s sold as a separate product for $40 and includes the original. The cost may seem a little steep at first blush, but the core of the game has been drastically expanded upon. Equal parts frustration and elation, XCOM: Enemy Within is a bonafide fist-pump producer, taking players through the same emotional peaks and valleys of the original. Like most expansion packs, Enemy Within is targeted towards the experienced. But Firaxis once again manages to balance XCOM's disparate elements, resulting in a challenging and satisfying experience. [7]

This review is based on early PC code provided by the publisher. XCOM: Enemy Within will be available on November 12th at retail on Xbox 360 and PS3, and downloadable on PC for $29.99-$39.99. The game is rated M.