There are three other places on the city map that are bad news for low level characters:
O Institute of the Arcane
The Institute is a long squat rectangular building that was used for testing the magical capabilities of Grey sorcerers. Magic was taught freely in the city to members of the scientific caste, but only a capable wizard could hope to pass through the gauntlet of magical tests, traps and puzzles, and become a full member of the Institute, with access to the deeper mysteries. If any PC magic users are capable of clearing the gauntlet, they'll gain the All-Seeing Eye and be able to pass into the abandoned Institute at will.
Near the entrance vestibule are teleport circles only activated by members possessing the Eye. Entrance to the Institute building itself requires passing a pair of Watchers requiring indigo colored gemstones.
The Institute houses a library of Essential Saltes (the blue dusts to resurrect long dead creatures ala "The Strange Case of Charles Dexter Ward"). The entertainment value of the players resurrecting long dead aliens from distant worlds is terribly exciting to me. I've got my Random Esoteric Creature Generator all ready to go.
The Institute can also be accessed from level 6 in the dungeons, the Greater Halls of Science.
P The Great Plaza
If someone were to follow the column-lined processional boulevard all the way through the city, they would arrive at a huge plaza, surrounded by religous ziggurats, and towering over the plaza is the Spire of Thaumaturgy. The only objects on the icy, windswept plaza are a pair of massive fifteen foot high dull iron boulders.
Unfortunately, once someone steps foot into the plaza without violet passkeys, the boulders unroll themselves and stand erect, unfolding into massive 4-armed Iron Golems with misshapen alien heads and bulbous eyes. They'll smash anyone trying to cross the plaza or step foot on the crooked stairs leading to the Spire of Thaumaturgy. A kind DM might allow the characters to see the crushed remains of the Golem's last victims out on the plaza, and retreat is certainly allowable.
Q The Spire of Thaumaturgy
"I have harnessed the shadows that stride from world to world to sow death and madness..."
The many floors of the spire are dedicated to summoning chambers, magic circles, and libraries of knowledge about beings "From Beyond". (I'm going with the D&D definition of Thaumaturgy as summoning, and not the historical meaning). I imagine the tower will be filthy with invisible, floating, amoeba-like monstrosities that leaked through an improper summoning circle somewhere in the tower.
The undead inhabitant of the tower is the ancient alien lich, Kar-Qo (descendant of the Ghost Scientist Xeph-Ka near the Plaza of the Watchers). Where Xeph-Ka failed to create the lich formula, Kar-Qo succeeded, attaining immortality and severing his connection to the hive mind; he "survived" the mass extinction because of his separateness and has spent untold millennia pursing different agendas, uncaring of the ruined city around him. However, he'll be upset at being disturbed by fleshy pink bipeds.
Besides the plaza entrance to the tower, there is an elevator available from levels 7 and 8 in the under city that ascends to the Spire of Thaumaturgy.