Short answers, pulled from the story.
Hideo Kojima took over the Metal Gear project in 1987 after a senior associate had already begun work on it. The original concept was a straightforward action game featuring modern military combat before hardware limitations forced a complete redesign.
Players controlled Solid Snake entering Outer Heaven without any weapons initially and had to find equipment like gas masks or binoculars scattered throughout the base. When an enemy spotted Snake, the game entered Alert Mode immediately requiring players to avoid guards rather than destroy them.
NES sold one million units within United States markets alone during its release window. It became second top-selling game nationwide behind Super Mario Bros Two in October 1988.
Clint Hocking designer behind Splinter Cell stated every stealth-action game owed existence solely to Metal Gear success. Cliff Bleszinski creator of Unreal and Gears Of War cited military themes plus action gameplay as major influences naming his work after original inspiration.
A Famicom port released in Japan on the 22nd of December 1987 without Kojima's involvement. Another Konami division received source code from Tokyo without original team consent while Masahiro Ueno led programming efforts under a strict three-month deadline.