After Defender became one of the biggest hits of 1980, of course Williams wanted a sequel. Williams also knew it would be best to have the team responsible for Defender work on that sequel. The problem was, Defender creator Eugene Jarvis had left Williams to form an independent development team. To solve that problem, Williams simply hired the new company to work on the sequel. In 1981, Jarvis and his team, known as Vid Kidz, delivered the commissioned sequel, and Stargate was released to arcades.
A couple of years after its successful arcade debut, Stargate saw home release on a couple of home computers, courtesy of Atari. In 1984, Atari also released the game for their Atari 2600. Then, something strange happened: Stargate ceased to exist. All further home versions, including the NES release and an Atari 2600 rerelease, were marketed as Defender II. This was also the name given to all official home releases of the emulated arcade game. The official explanation is that Williams wanted to ensure they had a solid copyright on the game's name, and the name "Stargate" had already been claimed for another product by another company. That product wasn't a video game, but Williams decided just the same that Defender II was a safer bet.
Defender has seen a few more sequels beyond this one, but Stargate remains the most enduring... no matter what the name!