Earth Defense Force is a horizontal shooter by Jaleco released in Arcades in 1991 and later ported to the Mega Drive/Genesis in 1992. It is best known for having an experience system which fills up depending on how many enemies are destroyed. It later had a remade port for the SNES.
In the future, an alien race called the Azyma Empire has launched an attack on Earth. Using a giant attack satellite aptly called the Orbital Buster, they plan on annihilating all life on the planet. To counter this threat, the Earth Defense Force sends out their latest XA Series fighters to destroy the Orbital Buster and repel the Azyma Empire.
Earth Defense Force is similar to other shooters in that one destroys as many enemies as possible without getting destroyed themselves. However, EDF employs a mechanic not normally seen in shmups, that is an experience system. EDf does not have any powerups whatsoever. Instead, the weapons and support pods increase in power by the amount of enemies destroyed. Getting rid of formations will add more experience.
The game also allows for two player simultaneous play. The fighters initially have three shields, losing one upon hitting enemy fire or the terrain. If a player manages to get max experience (level 5) and then proceeds to fill the experience meter, they will gain another shield. Although only 5 can be seen, there is no limit to the number of shields one can get. Upon losing all shields, the player's fighter will fall in a ball of fire and explode. After continuing, the player is given the option to reselect their weapon and after returning to play, a lightning field (similar to the boss intro) will appear and damage or destroy all enemies on the screen.
The SNES port, called Super Earth Defense Force, is a more enhanced version of the original Earth Defense Force. There are enough differences to consider it a major variant. Among the bigger changes are graphical enhancements, the lack of a second player, and new weapons.