Xcom Enemy Within is the first major expansion for Xcom Enemy Unknown, containing new gameplay additions and modifications, such as MEC Troopers, genetic modified enemy, a new enemy to fight (EXALT), as well as new alien types and council missions. Xcom Enemy Within also includes a lot more new maps, with older maps modified slightly to adhere to the new MELD game play mechanic, as well as additional multiplayer maps and brand new second wave options.
The additional game play mechanics does a fine job of being implemented, without altering the core aspects of the game. You will have to make a choice whether the MELD containers are worth the risk of your troops, and how you will spend the MELD you do gather. Do you create MEC Troopers, genetic soldiers, or a mix of both? It is up to you!
MELD is a new alien device feature on maps that apply to genetic modifications of soldiers, or creating MEC Troopers and their MEC suits. On the map there will be 2 sources of MELD, which must be reached by a soldier and then activated before the timer on it goes off, dealing damage to any nearby units. MELD is not an infinite source, as any missed opportunities will not be present if you visit the same map again. You can gather some MELD from destroying Mectoids and I believe Elite Floaters as well, but the gains is small compared to the MELD containers on the map. The MELD gives you a choice of risk versus reward, and it plays out very well. One MELD container will generally be close to your units, and the other one in a more remote location away from your units, begging the question of whether it is worth risking the life of your soldiers.
MEC Troopers are a unique addition to the game, allowing a more strategic options of heavier firepower, at the cost of permanently making a selected soldier a MEC Trooper. Any volunteer for the MEC Trooper program will have their limbs amputated and replaced with cybernetic limbs, which will allow them to operate the new MEC Suits. However, any MEC Trooper will never be allowed to use conventional weaponry, nor his human skill set again, and will be forever be a part of the MEC Suits, and will sport a more robotic voice. This sacrifice gives them access to the very powerful MEC Suits, allowing them to use heavy weaponry, such as mini-guns, rail cannons, particle cannons, grenade launchers, and so forth. There is three tier upgrades to the suits, which you will pick one option between each one. Do you pick a flamethrower or the Kinetic Strike Module, as you cannot have both and operates similar to regular human skill sets. They will also retain their stats prior to augmentation, so its best to reserve them for higher ranked soldiers.
Their original class prior to becoming a MEC Trooper also gives them a passive buff, and they will level up and gain new skill sets for the MEC trooper as they get promoted. They are great when you need to bring in the heavy firepower, especially against Mectoids and Sectopods, and other strong firepower units. You can also set them up for a support role, using restorative mist, and becoming a shield for other units, so the options are up to you!
Genetically modifying your soldiers with MELD opens up brand new strategic options to your soldiers. As you perform autopsies on the aliens, you will discover new modification options for the soldiers. There is 5 different sections that your soldier can undergo surgery for. Brain, Chest, Eyes, Legs, and Skin are the options available for modification as you discover new options. These options offer interesting game play choices, such as higher leg strength to jump onto buildings, Bio-electric skin allowing you to sense near by enemies without alerting them, and making yourself immune to strangulation (Seeker attack). You can even give his/her brain neural feedback, which will cause psi attacks against them to be repelled back to the attacker, causing a great deal of damage, and putting all their psi attacks on cooldown. So as you can see, you can great some interesting genetic soldiers, and they will change in appearance as well, because their arms will be more exposed in their armor. As with MEC Troopers, they will be out of action for a time (usually 3 days), except if you have a soldier with multiple surgeries prepped for him, then it will be 3 days for each surgery, and will be out of action until being released.
Exalt is a new enemy organization you will battle in Xcom Enemy Within. They are a terrorist organization designed to hinder and take down the Xcom organization, and are alien sympathizers, and will do whatever it takes to slow down the project. EXALT operates via cells in the world, and this opens up a new Covert Operations menu in the situation room. For a fee you can scan and locate EXALT cells, and send an operative inside to disrupt their work. Covert Op soldiers can only use a pistol and an item, and wear no armor, and when the time comes for an extraction, you must send in a squad to take down the cell and extract the operative, or he/she will be lost. Weaponry obtained from Exalt can also be used by your own units, and have the same base stats as XCOM created gear of the same type. For each operation completed against EXALT, you will gain new information about their base of operations and where it might be located. You can accuse a country of harboring them at any time, but making a false accusation will make country leave the project and add another mark to the Doom Tracker, so choose wisely! If you choose correctly you can launch an assault on their base of operations, and if successful, take them out of the fight entirely!
There is 2 types of extraction missions. One involves protecting the data in a “king of the Hill” style game player, where you must protect a square area with the computer inside from EXALT from hacking it and locating the other transmitters. The other mission your covert operative must hack two EXALT Comm Arrays and then proceed by getting the operative to the Skyranger Extraction Point.
XCOM enemy Within also includes two new aliens to battle, the Mechtoid, and the seeker. The Mechtoid is a Sectoid inside of a large bi-pedal mech similar to the MEC Trooper suits, and just as dangerous. They can even receive mind merges from other sectoids, giving them a shield above their normal hit points.
The Seeker is a squid-like floating enemy which can stealth and go invisible, and will then seek out lone units in order to strangle them. They will reappear and strangle their target, slowly killing them and making them unable to act while in the act, although there is genetic modifications and items that help against strangulation. They prefer “hit and run” tactics and will generally strike when you least expect it.
In addition to a base defense mission, and including Project Progeny (which was originally a scrapped DLC, but redone and included in Enemy Within), there is additional Second Wave options (options that can drastically change the game in a number of ways), changes in certain skills and level ups, and much more. New items has been added, such as brand new grenades, and brand new foundry changes, which works with the new MEC trooper additions.