Down Town low down

The crew discover a golden obelisk at the heart of the tower, with a large version of a signpost orb set into it that lights up and catches everyone: it mentally depicts the story of a race of technologically advanced aliens who had an empire that spanned many worlds and built new bodies for themselves... the bodies of the bugs! Then there was some large conflict, after which the bugs embarked on some grand new project that was supposed to protect them from further conflicts, but this in turn was followed by a vague & unclear disaster that sent the survivors running, looking for somewhere to hide where the force behind the disaster could not catch up with them: a vast system of caverns deep beneath the surface of a primitive planet...