I think the most interesting ancient superweapon story would be one where the weapon is just one more artifact and had nothing to do with the civilization's downfall. Like:
You're walking in the countryside in England or Syria or some similar place and you come upon a patch of smooth, flat stones all laid out neatly in a line. "Why, it's a Roman road!" you exclaim. How'd it come to be in this state of obscurity and disrepair? People used these roads well into the middle ages, so it would be silly to blame it on "the fall of Rome" as if the people here have some deeply ingrained cultural trauma related exclusively to roads. The truth is mundane but it's also touchingly melancholy; over time, as it fell into disrepair and as people found other ways to get places, fewer and fewer people used it until, one day, nobody did.
Now replace "road" with "giant space laser."