Vanguard is an early scrolling shooter. You pilot the Vanguard ship through several underground tunnels clogged with enemy ships. Each zone features different enemies and different natural hazards in the walls. Your goal is to reach the City of Mystery and destroy the final boss, Gond.
The Atari 8-bit port appears to be a conversion of the Atari 5200 release, but only exists as a prototype.
This page needs more screenshots.
The melodic synthesizer tune of Vanguard's energy theme was taken from the soundtrack of Flash Gordon without the permission. In particular, it comes from the song "Vultan's Theme (Attack of the Hawk Men)", composed by Queen. There are two versions of this file - short and long.
There are also 3 fanfares. It's unknown who composed it and where it's played in game.
(Source: Recognizable bars.)
Ripping Atari music is very arduous process so this is beyond the scope of this site.