Fun Fact: Metal Gear was ported from the MSX2 computer version, and was very different from the original. The overall level design and story were changed, and the player never fights the titular Metal Gear robot in the North American port at all.
The original Metal Gear was advertised on the back of comic books with layouts of all the weapons and gear offered in the game, but hero Solid Snake relied on stealth more than combat to infiltrate Outer Heaven and destroy the walking nuclear tank known as Metal Gear. This was the beginning of a franchise that would become a key part of video game history to the present day.
Metal Gear is often attributed as the inspiration for the entire stealth genre of video games. If an enemy saw Snake an exclamation mark appeared over their head and they would attack. The player had to slip out to another screen and escape. If the player triggered an alarm in any way, enemy soldiers would pour into the screen from all directions and Snake had to kill them all, or leave the building or that floor of the building entirely. While there was a huge array of weapons and equipment that Snake slowly accumulated, they were more for problem-solving than straight-up combat.
Snake had to find key cards to infiltrate buildings, and by rescuing hostages could increase his rank, which upgraded his inventory space and maximum health. The transceiver which managed radio communications was extremely important for conveying the story, and players could call various experts for advice by remembering their radio frequencies. Metal Gear also set the stage for the franchise's love of odd boss characters.