Per "Wyrms Footnotes," the sewers of Furthest are an appropriate venue for Gloranthan dungeon-bashing, as seen in Chris Gidlow's "Young Thrax" freeform and the Unspoken Word's "Tarsh in Flames" fanzine. Here are some background notes I cobbled together.
THE SEWERS OF FURTHEST
The City of Furthest’s sewer network began as a canal which drained the marshlands of the Oslir on which the planned Lunar city was built; as the city grew, this filled with waste from the growing population making the city increasingly unwholesome. Eventually, in the reign of King Phargentes, the canal was dug deeper and bricked over to prevent foul Air from rendering the city uninhabitable, and the modern sewers of Furthest were born: a grandiose civil engineering feat which serves to drain the marsh, tame storm-water surges, and keep the city and its streets clean and fresh.
The main construction material is the ubiquitous red bricks, sun-baked or fire-hardened, formed from the clays of Tarsh; building techniques are modelled on those of Dara Happan cities, benefiting from Carmanian and Sylilan innovations and the planned nature of Lunar Furthest.
The principal channel of the Furthest sewer system is the Cloaca Maxima, a barrel-vaulted (ie: semi-cylindrical) storm sewer about ten feet across, fed by smaller branching side channels. It runs beneath the central east-west street of the city, and empties into the Oslir River; when Rockwood Mountains’ snowcaps melt each Sea Season and the river rises, a backwash of fresh river water flows into the Cloaca (this is celebrated as the Holy Day of the Cleansing Oslira cult).
The most obvious “official” entrances to the sewer system are the Temple of Cleansing Oslira in the centre of Furthest, and the outflow into the reinforced Oslir embankment. There are, of course, others: private, secret and accidental. Side channels can be accessed from streets or basements, and occasionally interconnect with other underground features such as the catacombs, sangueducts, basements of the Royal Palace, etc.
The Cloaca Maxima has a walkway either side of the main channel high enough for a man to walk upright. To traverse any of the side channels, you would have to wade in foul water: this can be knee-high in Fire or Earth Season, but is usually waist-high from Dark Season through Sea Season, with dangerous surges when Orlanth rages overhead in Storm Season or the mighty Oslir floods each Sea Season. (The Cult of the Seven Mothers asserts that both storms and floods have abated since the construction of the Reaching Moon Temple, now that Lunar Tarsh benefits from the stability and balance of the Glowline; pre-Lunar records are patchy, but give some credence to these tales).
[INSERT NIFTY CROSS-SECTION DIAGRAM]
The sewer system is fed by streams (both above-ground and underground, some of them diverted), by waste-water from the public bath-houses, and by storm-water drains in the city streets: Lunar Tarsh has few of the aqueducts seen in the Empire’s heartlands, and there are no Mostali hydraulic pumps or other such anachronisms. Sewage enters via steeply sloping or vertical terracotta pipes (mass-produced locally), which connect the latrines found in the more luxurious private homes and public insula blocks to the system. These pipes are flushed by emptying water into them from rooftop rainwater tanks or pails filled from street fountains. Solid waste falls into cesspools, usually ten feet deep, where several pipes debouch: these are periodically excavated by the Earth cults for use as a rich fertiliser (usually in early Earth Season, when the water level is lowest), and reinforced lest they collapse. Liquid waste is washed away through the channels of the sewer system, eventually flowing (untreated, obviously) into the Cloaca Maxima and thence, the Oslir.
Dangers and Traps
There are few vents from the sewer system, so pockets of foul air and gases can build up over time as solid waste ferments in the cesspools, with occasionally nauseating or dangerous consequences. Sometimes pockets of stinking, unbreathable air belch upwards into buildings connected to the sewer system – a noble palace, perhaps, or a poorly-constructed apartment block. Those who travel the sewers of Furthest can recognise the foul stench of Orlanth’s Evil.
From time to time, poorly maintained cesspools collapse, sucking in unwary citizens as the ground below their feet gives way and drags them down to a vile death. Thus do the treacherous Dark Earth Goddesses of the Oslir Marshes claim their hideous revenge on civilisation.
It’s a sewer system, not a Royal Vault (in all fairness, it’s a sewer system connected to a Royal Vault), but nevertheless… from time to time, valuable items are dropped into the cesspools, or flushed down the storm-drains from the city streets above: this can happen inadvertently, or through the richness of noble cuisine (banquets may contain exotic spice-seeds, gold leaf, pearls, or other indigestible rarities which in due course are ejected into the cesspits), but sometimes the sewers are seen as an easy way to hide or dispose of unwanted items (cursed treasures, inconvenient corpses, or things too recognisable to be sold). Also, underworld figures can move freely through the sewers avoiding patrols in the streets above, and these thieves and fences of Furthest may well carry valuables with them.
The sewers have few permanent inhabitants other than vermin (eg: marsh-snakes, swamp-rats and rubble-runners), criminals and renegades, and the albino sewer monsters of urban legend (cannibal pigs, crocodiles, and the rumoured Block Cess Monster). Occasional unhappy Native Furthese militia patrols can be encountered, on punishment detail or special assignment, guided by representatives of the Union of Sanitation Engineers, a specialist guild of professional civil dungeoneers. Elite Royal Palace Guards clad in their crimson and gold panoply bar the entrance to the palace cellars; ghosts and the undead preserve the dubious sanctity of their catacombs, shrines and sangueducts; fugitives and other desperadoes generally prefer not to be disturbed as they go about their furtive business.
The only prominent cult associated with the sewers of Furthest is the Cult of Cleansing Oslira, an offshoot of the River Goddess’s Cult, whose gleaming, porcelain-tiled shrine in the city centre is built atop the main entrance to the Cloaca Maxima. Side channels connect the sewer system to at least one basement shrine maintained by White Moonie heretics, and a rumoured Blue Moon assassins’ death-cult temple (no such facility is officially supported by the Empire or the Royal House).
Some sources consulted:
Peter Connolly, “The Ancient City”
NB: this is mostly based on descriptions of Mesopotamian, Indus Valley and Roman sewer systems. Historically interesting: the reason for covering over drainage ditches running down the middle of the streets to create closed sewers was to hold back the stench of the malodorous waste-water: that is, defeating evil Air. You can see why Tarsh King Phargentes would be keen on taming this, after his victory over Palashee Longaxe and the Orlanthi rebels (whose descendents are today’s Tarsh Exiles).
ADDED: my frequent collaborator Chris Gidlow suggested adding more chrome from Minoan Knossos: the palace complex sewers are also a monster-infested maze under the city, where the King keeps ferocious creatures imprisoned? He also noted that we are in a Foucault's Pendulum world of Lunar paranoia: "who knows for what sinister reason the Lunars have ordered every single house in Furthest to be connected to a vast subterranean labyrinth?" (And if anyone ever says, "Calm down, it’s just a sewer system" - well, that's exactly what They want you to think...)