The player must navigate the main character, a military operative codenamed Solid Snake, through various locations, while avoiding visual contact and direct confrontation with patrolling guards. If the player is seen, the game enters the "Alert Mode." In this situation, Snake must escape from enemy's sight in order to resume infiltration. The method of escaping varies depending on the circumstances behind discovery:
If Snake is seen and only a single exclamation mark appears over an enemy's head, only the enemies in the player's present screen will attack and Snake can escape by simply moving to an adjacent screen.
However, if two exclamation marks appear over the enemy (or the player triggers an alarm by being spotted by a camera, infrared sensor or by using an unsuppressed weapon), reinforcements from off-screen will appear as well. Snake can only escape by eliminating all incoming enemies, going outdoors, or by entering an elevator.
The player starts the game unarmed, but eventually acquires numerous firearms (namely a Beretta 92F, an Ingram Mac-11, an M79 grenade launcher and an RPG-7V rocket launcher), as well as explosives such as remote control missiles, landmines and C4 explosives. Each weapon has limited ammunition or supplies, which can only be replenished by obtaining ammunition caches or additional supplies. A suppressor can also be obtained that allows the player to fire the handgun and sub-machine gun without making a noise. Weapons can not only be used to kill enemies, but also to clear obstacles such as hollow walls and electrified floors. Snake can also use his fists to punch and defeat patrolling enemies. Occasionally, if the player defeats a guard with punches without alerting anyone, the defeated guard will yield a single unit of rations or an ammo box that can be picked up. In addition to enemy guards, the player will also be confronted by mercenaries who will challenge the player to combat, serving as the boss characters.
The enemy base consists of three different buildings, with multiple floors (including basement levels) within them. The player uses key cards and other items to unlock doors and explore new areas. Each door will only open to its corresponding key cards. Information can be obtained by rescuing hostages being held within the buildings. After rescuing five hostages, the player's rank will increase by one star (with the maximum rank being four stars), allowing for increased carrying capacity and maximum health. However, if a hostage is killed, the player is demoted to the previous rank.
A transceiver is available for the player to communicate with their commanding officer, Big Boss, or one of the resistance members operating covertly near Outer Heaven (Schneider, Diane, and Jennifer). Each of Snake's allies specializes in a specific subject. The player must keep track of their allies' frequency numbers in order to contact them. All of the radio messages are dependent on the rooms where the player is located.
The MSX2 version requires the use of an MSX tape recorder (such as the Sony Bitcorder) to save game progress, or a Game Master II cartridge in order to enable quick saves via a floppy disk. Versions on later platforms eliminated these complicated requirements, as they featured standardized storage devices such as memory cards and hard disk drives.
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