By way of a change, here's my own campaign description of Vincennes/Deneb, for which I wrote up a very detailed description and history using the GURPS sourcebooks, based on what little canon information we have about the world. I did my best to make it an interesting place to visit.
Vincennes is, according to canon, a Pop A, TL 16 Industrial waterworld in a complex trinary star system. (A/899AA6-G) I decided it has a population of 11 billion, and one small continent - less than 10% of the world's surface area - that is scoured flat by constant hurricane-force winds caused by Coriolis effect and dramatic temperature differences during the world's 25 standard-day year. There are other inhabited planets in the system, but the largest of these only has half a million inhabitants, so has no significant importance compared to the 11,000 million on the mainworld. As such, it's a good example of an 'extreme situation' high population world.
Vincennes was originally settled at the end of the Long Night, and the colonists dug underground shelters on the world's sole continent. These are now abandoned historical monuments. After Imperial contact, foreign investment and technology transfers allowed the creation of cities under the ocean. These prospered, mining minerals from the seabed and extracting biochemical compounds from the abundant ocean life. The economy of Vincennes boomed, immigrants flocked to the world, and the population started to accelerate. The underwater cities became huge arcologies, linked by maglev railways in sealed underwater tubes. Over the centuries the cities expanded and spread until by modern times they form a single continuous city, thousands of kilometres in diameter, spreading in a circle along the continental shelf of the world's only landmass. Mining operations and aquaculture farms are spread through the rest of the ocean. Vincennes Downport is located on the surface of the continent - the only significant settlement to be found on dry land in the present day - but even so it's mostly underground. A supersonic maglev railway connects it to the underwater megacity. Since the construction of the flying cities (see below) the Downport is mostly only used by bulk freighters and industrial cargo shippers, not passengers or free traders. (As such, it's a good place to land your ship if you want to avoid too many prying eyes...)
The Civil War era saw major political disruptions, but after Vincennes recovered the planetary government decided to invest in diversifying and improving the planet's industrial base. In 710 they financed the construction of the Deneb sector's first flying city, in imitation of similar developments currently underway in the Imperium's core. This city, called Blish and housing 700,000 people, was both a prestige project, an economic stimulus and a showcase for Vincennes' gravitic technology and heavy industry, which were cutting edge for the period. Many more cities followed, financed by both state and private investment. Their chief advantage is that they can climb high in the atmosphere, to avoid the worst of the Coriolis storms, and then descend again when the weather calms. They also each act as starports in miniature; docking with the underwater arcologies to take on heavy manufactured goods and other supplies, then slowly climbing to orbit to act as highports for the tens of thousands of commercial starships that pass through Vincennes each year. In the present day there are 71 such cities, including one owned directly by the Imperium to act as the seat of the Subsector Duke, and another owned by Vincennes' own royal family and the planetary government.
Most of the flying cities specialise in light engineering, high-tech manufacture and service industries, while the underwater arcologies concentrate on heavy industry, aquafarming and mining and other dirty industrial trades. This has led to a strong cultural distinction; the underwater cities are crowded, relatively poor and lower-class in population, while the flying cities are home to the social elite and a thriving cultural and artistic scene. Many of the flying cities are built in fanciful shapes; elaborate geometric forms or fake Baroque castles or even stranger designs. They are a famous tourist destination throughout the sector, and for most offworlders, the flying cities are symbolic of Vincennes. Few tourists ever visit the underwater arcologies; many probably don't even realise they're there, and certainly never appreciate that the vast majority of Vincennes' population still lives there. Many of the permanent inhabitants of the flying cities don't think about the underwater arcologies too often either.
Like many people who live in vulnerable and crowded artificial environments, the population of Vincennes show a rather schizophrenic social attitude. Normally they are very conformist, restrained and law-abiding people; but put them in a place where they feel safe and secure enough to let their hair down, and they can go completely wild. Saturday nights downtown in a flying city are an unmissable experience... (In this, I modelled Vincennes on a mixture of modern Japan and New Orleans' French Quarter, or the Left Bank in Paris... The set-up between the flying cities and the underwater arcologies is also patterned on Wells' Eloi and Morlocks, Lang's Metropolis, and France before the French Revolution.)
