Basic Idea:

The feel and concept for Cambridge Below has been stolen wholesale from Neverwhere by Neil Gaiman. In this world, all cities have an underside: a strange and dangerous place which pulls people in, those who have fallen through the cracks in the real world, a place they can never escape, because people Above no longer acknowledge their existance - and if they do, they are in danger of being dragged under with them.

The system is designed to work with anything from about a dozen players; the more, the better. It is designed to run anywhere that the number of players comfortably fits, preferably somewhere they can set up stalls and generally create the atmosphere of a fair - The Festival, where sessions take place, is analogous to the Floating Market in Neverwhere. The rules of the Festival are No Guns, No Fistfights and No Knocking Over The Stalls.

There are five main factions in Cambridge Below:

The Rat-Speakers, who are the largest faction numerically, but have a reputation as an untrained rabble, barely able to feed themselve and hopelessly enslaved to the rats who are their masters;
The Bravos, who are barely cohesive enough to be called a faction but generally represent the interests of people who like killing people and those who know where to buy them;
The Troglodytes, who run the Cambridge Underground and design mad creations in their underground laboratories;
The Templars, squeaky-clean paladins who regularly fail to bring order to the chaos of the Underside but always come back for more;
The Fey, a loose alliance of the strange and different, most of which have something not quite human about them.

The Rat-Speakers:

One large golden rat controls the rat-speakers from its secret redoubt far beneath the city streets. They maintain a large network of rats, who carry messages and information, and a hive of Rat-Speakers, who are kept minimally fed and clothed in order to gather resources and trade with the rest of Underside society on behalf of the rats. Rat-Speakers are fanatically devoted to their cause and respectful of rats, distrustful of outsiders, and have a tendancy to kill people for disrespecting rats.

The rat-speakers that come to the Festival are amongst the more presentable of their kind. They bring vast wealth in raw material, and are often willing to loan out unskilled labour and storage facilities to those in need of them. They mostly buy food and mundane clothing to feed and maintain the swarm of humanity they represent, but also carry an eclectic shopping list which represents the things the rats have requested. Should you win great favour with a Rat-Speaker, they may teach you the art of calling a messenger rat, who will reliably carry your notes in confidence to their targets.

All Rat-Speakers get the Knack 'Rat-Speaker' for free.

The Bravos:

The least organised of the Underside groups, the Bravos are a number of feuding gangs and individuals, rather than an individual body. Being a member of the Bravos means that you know people, you have contacts, but it's everyone for themselves out there. A Bravo has only the respect they can win at the end of a knife, and knows all too well that anyone can turn on them at a moment's notice for a loaf of bread and a tankard of ale.

The Bravos that come to Festival are a mixed bunch; some are sent on behalf of their gangs, but most of them are on their own, looking for stuff for themselves or stuff to sell to other people they know. They are likely to have a bunch of favours ready to call in, and a good selection of vicious bastards they've been asked to find employment for. As for what they're looking for - things to kill people with, mostly. There's always a market for weapons and armour in the circles they move in.

All Bravos get the skill 'Fighter' for free.

The Troglodytes:

The smallest faction by number, the troglodytes wield undue influence for two reasons. First, they run the Cambridge Underground, the safest way to get from place to place in the Underside. Second, they build both the finest fortifications and the best machines to smash them down with. Most of the Troglodytes are tremendously hard to coax from their secret laboratories, but the rewards for managing it can be immense.

The few Troglodytes that make it to the Festival are normally driven there by the desperation of their fellows or their own desperation; they have come to buy food, raw materials, and arcane components required for their madcap schemes or those of their friends. With them they bring a wide selection of artefacts, and tales of even more outlandish devices hidden in dark storerooms waiting for the right customer.

All Troglodytes get the background 'Craft Stuff' for free.

The Templars:

The Templars attempt to uphold justice, do the right thing, and impose some kind of law on the chaos of the Underside. They believe in the supremacy of humanity above all else, which makes them particularly unpopular with the Rat-Speakers and not on the best of terms with the Fey. They have a regrettable tendancy to make speeches, rally the troops, and set themselves up as a police force. The areas of the Underside that they hold are the only places where the caves and tunnels are clean and well-lit.

The Templars attending the Festival generally know better than to attempt to impose law and order on it, but are always looking for people to convert to their cause, and the faction has an insatiable thirst for footsoldiers. They also tend to be hunting for raw materials. On the other hand, they are generally quite happy to trade away the favours they get given for healing people, and the mastercrafted armour and swords that their craftsmen produce.

All Templars get the Knack "Healing Hands" for free.

The Fey:

Competing with the Bravos for the title of 'least organised faction', the Fey band together because they are different. Most of them were once human, but their time Below has changed them, either subtly or not so subtly, and although this makes them powerful, the fear they inspire often turns to hatred amongst the more mundane.

The Fey that attend the Festival generally do so for their own inscrutable reasons, although they are often working for those even more powerful and inscrutable than themselves. They come bearing strange gifts from those who did not wish to attend, and seem to have no particular needs beyond entertainment.

Fey are the only PCs who can take more than one Knack at character creation, but they must have some kind of strange physical feature (oddly coloured skin or strange facial markings, fur, claws, horns, fangs, or something similarly prominent) which is somehow related to their supernatural abilities. They get one Knack of their choice for free.