Defender II was another name for Stargate.
When the folks that had the rights to "Stargate" (Movie, Series, etc) got wind of the new "Stargate" game name, there was a hullabaloo the upshot of which is William's Stargate must now be called, Defender II. Little late as the game was already all over the place with the Stargate name, but it's the thought that counts.
Also, I HAVE the only known exisiting Defender III game and it DOES in fact kill guys on neighboring arcade games. Pretty cool feature.