This is a free booklet given out by Goodman Games (makers of Dungeon Crawl Classics) for Free RPG Day. It details the city in which the three first 4e DCC adventures will take place. I got mine in pdf format on FRD and a hardcopy a week or two after at the gamestore. It is good. It is only 15 or so pages long but it really evokes the feel of the city. With a few changes, I think it would work for Wyndhaven (Ptolus also works well for Wyndhaven but Ptolus is a bit more fantastic than Punjar and my world has never gone too far into the fantastic).
The Boiling Rock prison has a counterpart in Punjar: Blackwell Citadel. Blackwell Citadel is built on an island off the city's coast and it hosts the worst criminals in a city filled with criminals. The "Blackwell" proper is the series of ever-deeper tunnels that the inmates excavate during their incarceration. These tunnels then serve as the cells for the newest prisoners, who will join in excavating the next deeper set of tunnels for the next set of prisoners, and so on and so on.
If the Boiling Rock keeps as its justification an active volcano, it makes no sense to tunnel into it. However, if the magma is sufficiently recessed beneath the boiling lake, and the "rock" sufficiently raised above the surface of the water, then such an excavation as the blackwell is feasible.