Defender is a legendary, side-scrolling shoot-em-up. The player is charged with the mission of protecting a group of humanoids, stranded on an alien planet, from alien abductors. The game's levels are occupied by a large number of 'lander' aliens that try to abduct the humanoids from the planet's surface and take them to the top of the play area. If it succeeds, the lander and captured humanoid will merge into an alien 'mutant'. If a lander is destroyed after it has captured a humanoid but before it mutates, the humanoid is released and falls towards the ground; it must be caught by the player and returned to the planet's surface before it falls to its death. If all of the humanoids are abducted or otherwise killed, the entire planet is destroyed and all of the landers change into mutants.
The challenge becomes more intense as the levels progress, with 'bomber' enemies dropping mines, and 'pods' which break into many 'swarmers', which also must be destroyed. If the player does not finish the level fast enough, the extremely dangerous 'baiter' enemy will emerge and attack. The player's ship is equipped with a laser and a limited number of 'smart bombs' (although their power is limited). There is also a 'hyperspace' function which transports the player's ship to a random location, but sometimes the ship is destroyed upon rematerialization. An on-screen scanner aids the player by showing the location of all humanoids and alien enemies in relation to the current position of the player's ship.
Along with Namco's seminal Pac-Man, Defender shares the title of 'Highest Grossing Video Game of All Time' and to date has earned more than one billion dollars. It's interesting to note that when the now-legendary shoot-em-up was first shown at a 1981 Chicago arcade machine trade show, it was deemed to be a flop due to its high level of difficulty. Arcade industry insiders confidently predicted that both Defender and Pac-Man would be commercial flops and that Namco's Rally X would be the next major arcade success. (arcade-history)
Here are a couple of PC games that are either direct remakes or at least inspired by Defender.