Undetailed Places
This short list summarizes places that are hinted at in Neil Gaiman's novel but which aren't detailed any further.
The May Fair
*The May Fair is an annual celebration which resembles the Floating Market but is far bigger. It is more of a festival than a bazaar; various people from the Underside may show off their special abilities and talents here. It always takes place on the eve of May 1st at a changing location. There is no truce in effect at the May Fair; security is handled by hired muscle.
White City
Whatever or whereever White City actually is, the Marquis de Carabas ended a war between the Earl's Court and the Raven's Court waged because of it. The Earl still holds a grudge against the Marquis because of it, as he lost a dozen men in the retreat. We don't know whether the Raven's Court was more happy with the result of the Marquis' meddling...
The Underside Line
*This line of the Underground is not part of the Underground of London Above. It is the nightmare of trains, the cheap ride to hell. Among all the Underground lines, it is the only one which is not part of the dominion of Earl's Court.
London Fog
Sometimes, in the deepest caverns and recesses of the Underside, thick, yellow, stinking fog wells up. This is what remains of the London fogs of old - ghosts of the pestilent stinking fog that was caused by coal fires and other pollution.
Pockets of Old Time
In various places on the Underside, pockets of old time remain; time which has not passed yet. This time may conserve relics from the past - even live ones.