Before the known era, before the commonly accepted beginning of human history, some people or mythologies hold there were other eras. Some mythic, earlier golden age of humanity. Those who visit these elusive shards know that some echoes of this era (or maybe these eras) still exist, a fragment of their former glory but still potent. In those worlds, wonder and sheer terror is the key. Great heroes battle primordial monsters and gods unremembered by history walk an earth which might have never truly exist. Some are a bit more down to earth and horrific.
On this world exist a city: sometimes lost ruins, sometimes inhabited.
Yeah basically, think Conan, Kull, Berserk and Exalted.