In case you can't read my handwriting, the main paragraph reads "Central mountain is the megadungeon. Ascending rather than descending. Leads up to the moon. The ancient escape route of the Old Ones."
A recap of Utolso Varos:
This, as the name suggests, is the last city on earth, and mankind has retreated to it as the world grows old and fades, and life for human beings becomes hostile.
Like Nessus or Viriconium, Utolso Varos is almost collapsing under the weight of its own history. It is many thousands of years old, and feels it - it is decadent, listless, and resigned, although it still possesses a faded and elegant sort of beauty. It is situated on an island in the middle of a great inland sea, and beyond that sea is the wild, dying earth...scattered with the ruins and remnants of the civilizations of aeons past.
The earth has become so old that its very existence has become tattered and frayed. Time passes slowly, and the light of the sun has become flat and dull. Alien spirits and demonic things from other realities slip through the decaying fragmentary boundaries between their worlds and ours. Those who can practice magic hoard it, as if it might protect them from the inevitable end of all things. Gradually the human race dwindles, and history turns its face away.
My development of this idea is that in the centre of the city is a mountain that leads all the way to the Moon. Realising that their earth was dying, the people of the ancient civilisation who founded the city built a pathway up to the sky in an effort to escape. They then mostly traveled up this mountain to the moon and disappeared (who knows where?), leaving behind them only tattered remnants who evolved into the society that remains there now.
The mountain to the moon is thus a network of tunnels and pathways which is populated by the belongings and sentinels of the Old Ones which they left behind on their great exodus. But, in addition to that, alien things which later discovered the Moon have been descending through those tunnels and pathways and to the city below, with unpredictable consequences.