Due to the sheer size of the Well, there is no one monolithic culture. At the same time, the size of the Well, along with the various environments, allows GMs to insert whatever cultures they wish. You can have anything from primitive stone age tribes (best found in the Escarpments) to highly-sophisticated Greek, Roman, or ancient Chinese civilizations. However, none of these should societies should be all that large (maybe no more than a scattering of city-states along one of the rivers in the Plains), as the Snake Men actively discourage anything that might upset the balance of power or prove to be a threat to their rule.
The exact diet of a race depends on who they are and where they live. Humanoids tend to be omnivores, although the Mithra have a preference for meat, while the Dryads live more on fruits and vegetables. Of the Chimera, the Naga and Undines prefer a protein-heavy diet (needed to fuel their large bodies), while the Satyrs are omnivores. As stated, the Drakkine are also omnivores, while the Snake Men are pure carnivores, and tend to devour their prey (normally small mammals and other reptiles) alive.
What this means for the GM is he can feature fields of crops (possibly surrounded by ditches, moats, thorns, or fences to keep out unwanted herbivores), herds of cattle and other livestock, hunting parties, fishing fleets, and whatever else seems interesting at the time. You can even work this into an adventure—imagine a party of hunters out to bag a mammoth or a sauropod, or a fisher fleet hunting a sea serpent!
Buildings in the Well depend on where they are built and what they are used for. For example, buildings in the Foothills and Uplands tend to be dug into the earth, since there’s little to no trees available for wood. So, you end up with homes formed of blocks of sod or peat, or dug into the ground and walled and/or roofed with stone slabs. Forest dwellers make homes of wood, since it’s far more plentiful. Jungle dwellers tend to live in the trees (often literally), to keep away from ground-based predators. Along the Plains and Shore is where you might see mud-brick, fired-brick, and stone construction.
The size and scale of such structures varies widely. In the Foothills and Uplands, most homes are small, although the Gnomes are noted for the size of their buried tunnels and mines. Forest dwellings can be quite large, as there is a great deal of timber to work with. The same goes for Jungle dwellings, as bamboo is plentiful and easily worked with. In the Plains one can see full cities, complete with walls and moats.
Curiously, the entire Well is littered with the ruins of older buildings. Often immense edifices of crumbling stone, the ancient structures look to be temples, walls, aqueducts, market plazas, and so on. Many are used by the current residents of the Well as a basis for their own building efforts. These ruins can extend down into the Ring Ocean and are often pressed into service as docks where they do.
Different communities within the Well practice various forms of trade. The most common is the flow of metal and metal goods down from the Foothills and Uplands in exchange for foodstuffs sent back up. Rivers also form major trade routes, with rafts and shallow-draft vessels making their way up and down the waterways. Larger ships ply the Ring Ocean, exchanging goods from across the Well.
The goods being traded tend to be raw materials (hides, timber, metal ingots), perishables (grains, wines, smoked meats, cheeses), or certain finished items (knife blades, axe heads, wagon wheels, and so on.)
The languages spoken in the Well depend how much effort you want to expend on communication. In other words (so to speak), the PCs may find they can speak with everyone in the Well just fine, or everyone in the Well speaks an odd version of a known language (Greek, Latin, ye olde English, and so on), or they’ll need to learn at least one new language. Of course, if you go with the first option, it doesn’t mean the people of the Well speak English... it might mean the PCs can speak whatever the local language is automatically (which could be a sign of Snake Men meddling!)
There is no set religion in the Well, although the Snake Men tend to encourage worship of themselves as the (not so-) benevolent “gods” of the Well. At the same time, GMs are free to populate the Well with all sorts of real world and mythical religions and beliefs. If you already have ersatz cultures, you can easily have them follow an ersatz religion based on the culture. So your Native American knock-offs can worship various nature spirits, the Roman knock-offs offer sacrifices to the sky god, and the Aztec knock-offs kills prisoners to honor their divine overlords (guess which group captures the PCs?)
Although it may look like an idyllic pleasure park, the Well can be (and is) a violent place. Partially because of the fauna, partially because of the broad mix of humanoid types, and mainly because the Snake Men like it that way. Of course, this doesn’t mean the Well is in a constant state of siege, just that anywhere you go, violence tends to be the answer to life’s problems. This violence can take may forms: formal duels between designated champions; raids from upper level rings to lower rings; pirates on the Ring Ocean, who attack shipping and costal villages; fights over water, land, and resources; small parties who go out stealing horses, cattle, and wives (or husbands), and so on. This gives plenty of opportunities for the PCs to get involved in all sorts of mayhem in the Well.