This is the layer of ruins, a place of hard labour under a blood-black sky. Granite and wasted cities scatter the surface of the land, and polluted canals clog up and spill effluvium across the scarred surface of the layer.
This layer's a druid's nightmare because everything natural's been defaced or destroyed in some way. Strip mines belch filth into the air, slag heaps burn whatever stunted trees might have once survived in this layer, filth litters the poorly maintained roads crisscrossing the land, and crumbled ruins demonstrate the imperfection of structures. The petitioners here continue to defile a land which once, eons ago, might have been beautiful. They dig, without tools, deeper into the scarred land, looking for more stone and mineral to carve out a new city for the archduke of the layer.
The archduke, apparently, is never satisfied with his cities. Instead of forcing renovations, he demands entire new cities be built to satisfy his whim, passing his messages along through his pit fiend servants. He leaves the old cities standing to serve as abject reminders of work already performed and scorned. The ancient cities might have displayed glory and triumph long ago, but the abilities of the petitioners to build such structures any more have been beaten out of them. The archduke is a harsh taskmaster, devoted to making life miserable for others. After all, what's power for?
The old cities are homes for petitioners who've fled their masters, native Baator creatures who've lost their territories, and beasts from other planes who've found their way here. The cities are often cannibalized for stone to construct new cities. The city currently under construction, Malagard, is the best that anyone who saw the old incarnations can recall, but they're certain it's not going to be good enough to please the archduke.
Beneath all these old cities and into the slag mines lie tunnels honeycombing into the layer. Petitioners, whether lemures or more intelligent creatures, avoid these tunnels. Even lesser fiends don't tread there, for fear of what's rumoured to lurk in the passages. Greater baatezu generally don't travel there either, though they say it's because they already know what's down there and it holds no interest for them. Maybe that's the case and maybe it ain't, but something down there's got most fiends pretty frightened, and no one seems to know what it is. The cant is tat it's one of the original denizens of Baator, even though no one knows what they are.
Besides the city of Malagard, there's another burg or two in the layer. None of them approach Malagard for magesty or size, but they've got functions nonetheless. The most important of these is the city of Grenpoli, a training ground for betrayal and treason.