Gameplay
Throughout the games the player accesses computer terminals through which he communicates with artificial intelligences, receives mission data, and gets teleported to other levels via "Jump Pads". Though contact with computers is how they are primarily utilized, they are a fundamental storytelling element; some terminals contain civilian/alien reports or diaries, database articles, conversations between artificial intelligences and even stories or poems. Messages may change depending on a player's progress in a certain level. The ultimate goal of most levels is not to merely reach the end but to complete the type(s) of objective(s) specified: extermination of all or specific creatures, exploration of a level or locating an area in the level, retrieving one or more items, hitting a certain "repair" switch, or preventing half of the civilians from being killed (a mission only present in two levels in the first game).
Most levels contain platforms, defined as anything able to change its height. Though it is generally used to describe lifts, doors are included in this category. Doors may or may not show up on the player's automap and are usually opened with the action key. In cases where they are damaged or locked they can be opened by special designated triggers or switches. Switches control various functions such as lifts, doors and lighting and come in the form of manual switches that can be toggled with the action key, stations for computer chips or breakable circuitry. Some switches are "tag" switches that execute multiple functions at once or those that must be activated as part of "repair" missions. Another notable level feature is teleporters, able to send players who use them to different parts of a level or to other levels. Aliens are unable to use them.
As the player combats enemies, he will inevitably take damage and must replenish health by means of special panels that recharge his suit's shields. There are three types of such panels, recharging single (red), double (yellow) or triple (purple) shields. Occasionally a full "color bar" of shield power can be recharged instantaneously by obtaining a powerup canister.
In Marathon 2 and Marathon Infinity, the player can swim in four different types of media: water, sewage, lava and acid/plasma; the latter two are damaging to health. Levels of the original Marathon did not contain media capable of swimming in. However, some did have floors textured with orange lava or green goo that will inflict damage on the player when standing upon them. When the player is submerged in liquids, the run key can be used in order to swim. In liquids or in "vacuum" areas, the player's oxygen depletes and it must be recharged using a special oxygen recharge station. Should the player lose all oxygen or health, he dies and is sent back to the last pattern buffer (a special terminal that according to the storyline saves molecular data) at which he saved. Because some levels do not have these devices, dying results in having to complete the entire level again.
Gravity is fairly low on some levels, and the correct application of the flamethrower or alien weapon allows the player to hover. "Hopping" with the grenade launcher or rockets can be used, but usually involves a fair amount of damage to the character.
The heads-up display has an inventory, health and oxygen bars, and a motion sensor. The motion sensor displays alien creatures as red triangles and friendly humans or robots as green squares; it tracks their motion relative to the player, represented by a square in the middle whenever the player moves. The brightness of the middle square represents how still the player is and how well he can be tracked. On some levels the motion sensor is erratic due to magnetic artificial gravity fields.
Marathon has five difficulty settings: Kindergarten, Easy, Normal, Major Damage, and Total Carnage. Differences involve the omission of some creatures from each level and creatures marked as minor in the game's physics model are promoted to their major versions or vice versa. On higher difficulty levels, creatures attack more frequently and have more vitality and on the highest setting (Total Carnage), the player is allowed to carry an unlimited amount of ammunition.
Read more about this topic: Marathon Trilogy