XCOM: Enemy Within replaces Enemy Unknown's opening Arthur C Clarke quote with a pearl from American polymath R Buckminster Fuller. “Those who play with the Devil's toys will be brought by degrees to wield his sword.”

It captures the central theme of this major expansion: how far are you prepared to warp your soldiers with alien tech before they lose touch with the race they're defending? Fuller's words do fall slightly askew, though. It's not a sword, it's a giant rocketpowered robot fist.

That fist belongs to Rebecca 'Freya' Berry, a US soldier strapped into a ten-foot-tall mech suit. I'm playing through a battle at Firaxis HQ, Maryland, surrounded by a squadron of design leads, all quietly waiting to see what I'll do with this destructive new toy. The XCOM fundamentals remain unchanged: you build a base, research alien tech, conduct autopsies and invent new gear for your soldiers to utilise in turn-based human vs alien skirmishes. Enemy Within introduces new enemies, units, classes, gear, abilities, maps and more to grow that base game into something bigger, tougher and full of giant robots.

Mech-Freya is standing next to a tractor in the dark. I'm using my robo-soldier as a unmissable lump of bait, but I've played XCOM. I know how this goes. You throw a lone soldier forward and before you know it they're knee-deep in little grey men blasting them in the flanks. Not today. I select my nearby assault soldier, Marco 'Maestro' Bruno, and have him take a covering position in the back of a pickup truck a few feet behind Freya. To my delight, he confirms the order in Spanish. Soldiers speak their native tongues in Enemy Within, rather than the generic US voices of old. XCOM's team of international super-warriors will finally sound that way.

Is it Marco's Mediterranean machismo that attracts the aliens? Is it the red glimmer of his laser shotgun? Only the hive mind knows, but a pair of Sectoids dart nervously into the light like rabbits in a Attenborough documentary. A hulking shadow follows them. Is it the alien close combat specialist, the monstrous Berserker? No. It's another Sectoid, in a tall, sleek exoskeleton of his own. That's new. He looks both fearsome and adorable, but that won't save him. I'm going to make my big thing punch their big thing, for science.

Freya lopes forward with surprising grace and engages the enemy robot. The 'Mechtoid' tries to grapple his attacker, but he's having trouble processing the fact that a pair of rocket thrusters have just popped out of Freya's right hand and started firing. With a crunch, the human mech pilot pounds the alien into the earth. Firaxis have gone to town on these new animations. Mechs will mercilessly obliterate enemies of all sizes in close combat, if they have the correct upgrades. I'm told that the killing blow can even knock their targets back a distance. They can punch enemies over precipices. They can punch enemies through walls, creating openings and shot opportunities. They can punch enemies into cars to blow them up. I like them already.

Mechs serve as an entirely new class. You'll need a new Cybernetics Lab to construct them, but once you've got that, you can convert an existing soldier into a robo-soldier. They'll gain a passive benefit based on their former role – assault soldiers will be more resistant to close combat damage, for example – but their skill tree will be replaced by a new robotic one, and their equipment slots will only be able to hold new mech gear that can be researched by your scientists. Offensive gear includes the aforementioned doom-fist, a grenade launcher and heavy weapons, which – like the standard XCOM weapon variants – come in three tiers: minigun, railgun and particle cannon. They can also equip other gadgets, such as jet boots to get them to higher ground, and a healing mist that helps out nearby allies.

Back at base, the augmented forms of your mech soldiers will wander around outside of their suits, sparing us the sight of a man trying to eat breakfast with a Gatling gun for an arm. Just because they can detach themselves from their battle shell doesn't mean they aren't disfigured. Holes have been carved into their bodies to allow for the interface implants. When you commission a mech conversion, you're asking a soldier to sacrifice their body to your cause, a morally troubling scenario that I imagine Firaxis enhancing with some grotesque conversion scenes. “Such scenes may or may not be in the game,” laughs lead designer Ananda Gupta.

I ask Gupta how far the moral ambiguity goes. It's potentially a significant tonal shift from Enemy Unknown which, in its broadest strokes, was a positive story about international unity and the power of human innovation. “We definitely did not want to go very dark,” he says. “You are still the good guys, you are still XCOM, you're still defending the world against the alien invasion. You are the last hope. But we also wanted to make it clear that when you get this expansion pack you'll be asking more of your soldiers than you have ever before.”

I'd buy that if it was just about the mech conversions, but there's another breed of soldiers that apply the 'enemy within' theme quite literally. I scan the battlefield and find my sniper, Thomas 'Tireur' Durard, a genetically modified human fused with the processed flesh of captured aliens. He's hard to see thanks to the camouflage biotech implanted in his skin, but he de-cloaks as I move him to an elevated position, and I notice that he's wearing new armour. GM soldiers have visual variants for all of Enemy Unknown's armour types to set them apart from standard plods. They're still human, to outward appearances, but those alien molecules grant them superhuman abilities. When Tommy McAlien reaches his destination, he leaps ten feet into the air to reach his sniper perch, the show-off.

Like psychic abilities, genetic modifications are also applied to a trooper's class skills, added to their brains, eyes, skin, leg and chest regions using material recovered from autopsies in a new Genetics Lab. Scarily, human soldiers can be both psychic and genetically modified, opening up a huge number of customisation options for your most powerful soldiers.

I'm just starting to feel sorry for the aliens when another Mechtoid stomps out of the gloom. One of his flanking Sectoids hangs back and bestows a pink psychic buff on the bot, granting him an extra few bars of health. Those vanish immediately when my GM sniper shoots the enemy mech's little helper in the head. Then I have Freya hit the mech really hard with her huge metal hand. My heavy weapon guy, Edmundo 'El Cid' Ramos goes for a killing blow, but fluffs the shot and sprays plasma into the sky. It's all down to the Marco to finish the job. It's a 38% chance to hit, but his aim is true. The Mechtoid collapses. Marco cocks his laser shotgun one-handed and drops a quick Spanish curse on the corpse.

The turn isn't over. There's one Sectoid left in view, hovering uncertainly near a forklift. You picked the wrong farm to accidentally crash land in, buddy, Edmundo might say, in Spanish, if he had any action points left. I mobilise my ace in the hole, a second mech piloted by a laid-back German fellow wielding a railgun and a huge flamethrower. There's only enough fuel for a couple of sprays, but I select the flamer, angle a large red aiming cone squarely over the little survivor, and watch the mech douse the entire area in fire. The Sectoid is vaporised instantly. Everything touched by the attack is charred, and remains on fire for a few turns after.