Gameplay
The game takes place within two distinct views, called the Geoscape and the Battlescape. According to GameSpy, "Playing it again in 2012, it comes off as both completely brilliant and slightly insane. In effect, X-COM melds an SSI Gold Box RPG with a highly detailed 4X game like Master of Orion, making it in some ways two entirely different games."
The game begins on January 1, 1999, with the player choosing a location for their first base on the Geoscape screen: a global view representation of Earth as seen from space (displaying X-COM bases and aircraft, detected UFOs, alien bases, and sites of alien activity). The player can view the X-COM bases and make changes to them, equip fighter aircraft, order supplies and personnel (soldiers, scientists and engineers), direct research efforts, schedule manufacturing of advanced equipment, sell alien artifacts to raise money, and deploy X-COM aircraft to either patrol designated locations, intercept UFOs, or send X-COM ground troops on missions using transport aircraft.
Funding is provided by the 16 founding nations of X-COM. At the end of each month, a funding report is provided, where nations can choose to increase or decrease their level of funding based on their perceived progress of the X-COM project. Any of these nations may quit, if the nation's government has been infiltrated by the invaders. Through reverse engineering of recovered alien artifacts, X-COM is able to develop better technology to combat the alien menace, and eventually uncover how to defeat it.
Gameplay switches to its tactical combat phase whenever X-COM ground forces come in contact with aliens. In the Battlescape screen the player commands his soldiers against the aliens in an isometric view and turn-based battle taking place on a semi-randomly generated map. The combat features a system of action points that can be spent for movement and a variety of actions (through an icon based GUI), such as managing equipment, picking up or throwing objects, kneeling down, using items (such as medi-kits) or priming hand grenades. The points can be also spent on managing ammunition and discharging firearms, using either a snap shot, an aimed shot, or possibly a burst if a given weapon has an autofire mode (some weapons can also have alternate types of ammunition loaded, such as high-explosive rounds or incendiary rockets). There's also a feature called opportunity fire, enabling combatants to automatically shoot at a spotted hostile during the enemy turn in case if enough of their action points have been reserved for this.
One of three mission outcomes is possible: either the human forces are eliminated, the alien forces are neutralised, or the player chooses to withdraw. The mission's score and result is based on the number of X-COM operatives lost, civilians saved or perished, aliens killed or captured, and the number and quality of alien artifacts obtained. Troops may also increase in rank or abilities, if they made successful use of their primary attributes (e.g. killing enemies). Instead of experience points, surviving human combatants gain points (a semi-random amount depending on how much of the action in which they participated) for their attributes, such as Psi or Accuracy. In addition to combat personnel, the player may use unmanned ground vehicles, outfitted with heavy weapons and armour but not gaining experience. Recovered alien artifacts can then be researched and possibly reproduced. Captured live aliens may produce information, possibly leading to new technologies and even an access to psionic warfare.
One reason for the game's success is the strong sense of atmosphere it evokes. Soldiers are vulnerable to alien attacks even when armoured, and the use of features such as night-time combat, line of sight and opportunity fire allows for alien sniper attacks and ambushes. The enemy comes in numerous forms, and players run into new, deadly aliens repeatedly without any knowledge of their characteristics and capabilities beforehand. The course of skirmishes is also dictated by the individual morale levels of their participants on both sides; a low morale can result in them either dropping their weapons and fleeing in panic or going berserk and opening fire indiscriminately. At night, the environment needs to be illuminated by flares or fires or else the humans can only spot their enemies at a very short range.
Fan-made patches fix a notorious bug which results in the game always resetting to the easiest difficulty level ("Beginner") after completing the first Battlescape mission, no matter what difficulty level has been selected. This glitch was not noticed by MicroProse and was not fixed in the official patches, resulting in the very high difficulty of the sequel due to many complaints from veteran players who believed that the original game was still too easy even on seemingly higher levels. IBM Master Inventor Scott T. Jones' noted 1995 patch-turned-mod, named XComUtil, fixes it as well as addressing many interface problems and better balancing the game; in 2010, a task of its further development was given to David Jones.
Read more about this topic: UFO: Enemy Unknown