I watched this anime in the late 80s.
The protagonists may have been kids (perhaps a boy and a girl). I seem to recall them either crash-landing on an unknown planet, and constantly trying to outrun large spherical (probably red-coloured) sentinels that were trying to kill them. I think the sentinels were reminiscent of Cyclops, with lasers that were coming out of their eyes.
In their quest, they understand the history and why they were being hunted they discover a long-lost underground city, with no one alive. It culminates in finding a room/hall with a weapon (looked like a rocket launcher or something). Leading up to a weapon was a dead body whose last moments may have been trying to reach it.
They get the weapon, but the video tape ended.