Imagine a city where the primary population is an avian race. Well assume for the sake of argument that these birdlike creatures have "hands" as well as wings which allows them to develop and use at least a Victorian/early 20th century level of technology.
They are also approximately human sized.
What sort of differences would their city have?
Because there is little risk of injury from falling, the creatures would not suffer from vertigo. Our skyscrapers have walls around them to stop people from falling. Non-structural guards would not be required.
Birds quite happily perch in locations that would be terrifying for a human. A pigeon will sit on the windowsill of a skyscraper. Perches could be used instead of benches. Platforms could be used instead of rooms. Poles could be used instead of corridors. Railings and fences are less necessary because a fall is likely not harmful.
A platform at the top of a pole would be a viable habitation, as would a hanging sphere. A multilevel dwelling might consist of a series of pods hanging from a strong wire for example. You would need no physical way to move between levels, just space to fly. Open spaces could be left within a building to allow access to other levels. A vertical surface with perches would be as useful as a horizontal surface.
A city on the side of a cliff would be possible. The inhabitants could reach holes or perches without stairs or lifts. Undersides of surfaces could be used, perhaps the underside of a large tree branch, cliff overhang, cave ceiling, or the underside of a larger dwelling.
Exit holes could be on any side of an enclosure, top, side or bottom. An exit hole at the bottom would keep the wind and rain out effectively. Assuming the creatures sleep standing up this might be a good option. Furniture could be mounted on the wall and accessed by means of perches.
A human habitation has large rooms with a flat floor which we walk about on. An avian dwelling could be spherical, could be made of multiple platforms at different heights, or could simply be a network of poles.
Our property laws are based in two dimensions, you own land. An avian race might own airspace. A dwelling could be created at any height provided it could be elevated on poles or suspended by wires at that height. Dwellings could be suspended from the branch of a large tree for example, or cantilevered on a pole from a cliff.
If the species has evolved roosting in trees, motion in the wind might be acceptable. Multiple Dwellings suspended from a single long wire might move and turn in the breeze.
A flying creature is more easily burdened than a walking creature. Items of furniture would need to be lifted into position. Day to day tools, machinery, electronics, implements and clothing would need to be kept lightweight.
If cities are high, building material would need to be lifted. Cranes would be possible, but older buildings might be made out of small, light blocks, light wooden beams or fabric. Modern buildings might make use of light weight metal or plastic lattice.
Building from stones would be difficult and expensive. Heavier items might symbolise wealth. Perhaps high status building might be made out of heavier materials such as stone or steel sheeting.
If you're high up, dropping a tool probably means you lose it. You might even hurt or kill someone. Items might be made to be hooked on or otherwise attached to surfaces. Hooks, clips and cleats might be commonly found on clothing or walls. Think about a ship in a storm, everything has to battened down and attached.
Flying is energetic and requires big body movements.
What happens to the elderly, infirm, or just people who are feeling tired, or if the wind speed is too high? Are there laws that buildings must be accessible without flight? Perhaps public buildings are connected with wires. You might find streets made of wire mesh slung around the city center. Old parts of the town might be less accessible.
In the same vein, running has been considered a low status activity in some societies. It's tiring. Servants and slaves need to run around to get their jobs done. The rich can afford to have people run about for them.
Perhaps flying might be a low status activity. It takes a lot of energy, and if you're rich enough you can afford to have ladders, lifts, perhaps even a litter or aircraft.
A bird has one defence, flight. A bird on the ground is a sitting target. It's bones are thin and light, it stands little chance against a predator. Intelligent avians would be lightly built and armoured, and might in the past have had to deal with predation, just as humans did.
Much as humans have an instinctive fear of dark confined spaces where predators might hide, intelligent avians might have a fear of low down accessible places. They might feel more comfortable in clifftop locations.
A natural form of punishment would be clipping of wings. This would hobble (and possibly shame) the avian for several months or years. Breaking of wings would achieve a similar, more long term effect. You might expect to find such outcasts living at ground level.
It's easier to take off into the wind. Mobile dwellings might rotate so the egress faces away from the wind. There might be a launch perch on the reverse of the dwelling so the inhabitant could launch into the wind. Perhaps there might be a feathered projection on one side of the dwelling to turn it into the wind.
You would probably see weather vanes dotted around.
Making a cooking fire would be difficult and possibly dangerous without a stone floor to build it on. Fire stones would need to be present in buildings if the creatures make use of fire.
Heavy industry at altitude would be difficult. You could expect materials to be lightweight, foams, plastics, plywood, aluminium with holes (though mining and smelting would be difficult), wooden lattices, dried funghi or bone, stabilised mud, perhaps printed structures.
Perhaps rain water could be collected in an arial reservoir. Perhaps it is pumped up, or perhaps tree sap is tapped and used. Perhaps individuals could have their own reservoir, maybe a leaf bent and pinned.
Lifting water for flush toilets would be impractical. The creatures might perhaps land to defecate, do so on the wing in special areas, or simply fly outside the city limits. Private bathing and toileting would be difficult. Lower levels would probably smell rather less fresh.
An unmarked wire would be the equivalent of a big hole in the road; striking it would cause severe injury or death. Expect lifting wires under tension to be marked somehow, perhaps with lights, coloured flags at intervals, or just light coloured rags.
Just as different parts of a modern city like London use different architectural styles, you could expect different regions of your city to have different architecture as technology has changed over time. Very old buildings might be built of stabilised mud and branches. Modern buildings might even be 3D printed.
Walking and driving would feel un-natural. Perhaps the society might use ground based transport for moving heavy loads, or perhaps airships.
Flying creatures unlike bipeds can travel very quickly over comparatively great distances so short range transportation of the type we have (subways, taxis) would not be useful. Longer trips would require some means of transport, presumably airborne.
A human transport is designed to mimic the feeling of being in a room. Walls, ceilings, chairs, little tables. This might not be the case for an avian species.
A person can fit into a space small enough to stand upright. A winged creature might feel uncomfortable without the ability to take flight. Space would need to be left between structures to allow flight. Doors or gates would need to be wider to allow landing.
A secure structure would need to be completely enclosed. Fences would only be effective against ground based animals. Likewise for a private structure.
At altitude, wind is likely to be stronger and more reliable. Expect wind turbines to be used for electrical and mechanical power.
As a side answer, it might depend on their flying style, not all fliers in nature are alike. Many birds need room to maneuver and the right air currents, while dragonflies are able to hover and dart in at smaller angles. Different flying styles might have different castles.
For instance, dragonfly like people might live in a narrow canyon that they could easily hover down into, with small helicopterlike landing pads. Batpeople might build upside-down cities that cling to ceilings. Predatory bird like people might go for Avatarlike Airbender cities with more open approaches and longer landing pads. Or if they're more like hummingbirds, only perches or places that they only touch down briefly.
Adding to superluminary's great answer. I would expect mountain sides and cliff faces to be rather high demand for home locations. Like the cliff dwellers in the American Southwest, it would be a very safe location from predators and even reasonably easy to protect from enemies. With out having to worry about ladders or stairs it would also be an efficient use of space for more advanced groups, fewer homes using up arable land. When actually building in other locations I would expect most building to be rather tall, internal stair cases and elevators wouldn't be needed. A crane on the top that can hang over any side would be sufficient to get heavier things up to floors since every flat could have big open doors directly out to a landing.
If you also think about bird's nesting habits; like where, why and how they build them, that might give you a direction to start in. For example, many birds will build nests in places that are difficult for predators to reach. For a suitably advanced avian race, this may not be necessary anymore, but it might be a cultural hang-up that they continue to observe.
An avian society may also live as close as possible to their preferred food source - whether that's farmed grains on the ground (perhaps tended by enslaved sub-species or ground dwelling creature) or some kind of prey.
Another interesting thought, spurred by bowlturner's answer relating to cliff's being defensible, is that if an avian culture is territorial or warlike, it is likely that at least some of their enemies are also avian. Rendering cliffs no more defensible than anywhere else. For a city builder of such a culture, you might choose to design certain parts of the city to impede flight or restrict freedom of movement to make attacking more difficult.
There’s been some great answers to this question, however there’s a few specifics that probably should have been initially clarified, starting with if these animals are living in an Earth analogue environment, what kinds of environment(s) they occupy, and if they are able to fly or glide. (Some avian species are unable to do either e.g. penguins).
These questions aside, I’ll assume you’re imagining a humanoid species capable of flight in an Earthern world:
I would actually argue that simply being occupied by a species capable of flight would have very little impact on the city. The defining characteristics of the city would have more to with the environment, economics, and the culture, than just anatomy. While flying is novel, it’s implications on the essential necessities of a Victorian Era industrialised city are very limited, therefore their cities are unlikely to look radically different.
Transportation
While flying (or rather gliding) is highly efficient for long distance travelling, the ability to transport large / heavy objects is acutely limited. Ground transportation would be an essential and integrated feature of the urban environment by necessity. A train or barge is the most efficient way to move tonnes of iron ore, regardless of wether you fly or walk on land.
Aerotechnology however, would most likely be accelerated. Steam can’t effectively power aeroplanes, but it can be used by balloons and blimps, which would be superior methods for long distance air travel based on their ability to carry large payloads and travel at consistent minimum speeds. Gliders would be another option.
Sciences including cartography, physics, meteorology, mathematics and engineering might also get a boost.
Given they are probably going to be weaker than humans, technologies that allow these lighter bodies to use more leverage are most likely ubiquitous.
Architecture
Stairs, elevators, escalators etc. would all be common - imagine being expected to walk up and down the stairs in every skyscraper just because you have legs capable of doing so! Given vertical flight is all about hard flapping, it would seem less efficient to fly than to walk up and down inside a tall building. Either way, elevators and funiculars are still more practical than flying or walking steep or high points. Buildings most likely have stairs and handrails too as they need to be safe and accessible for the very young, the sick, the old and the disabled.
Urban Layout
It’s quite likely the city would be orientated to prevailing winds, and organised in such a way to provide access to those winds. Likewise if this species has some sense of the magnetic field the city may align to that as well, or instead.
It’s no more reasonable to assume an avian species would live on cliff sides or in glorified nests, than it is to presume humans would naturally live amongst the tree branches just because their primate cousins do.
Your question asks what impact the ability to fly would have on the nature of an industrialised city. I would argue that the ability of citizens to fly has little impact on the essential nature of an otherwise human city. The differences are more likely to be superficial, and reflect what is economically, technologically and culturally permissible - rather than being determined just by biology.
Industrial cities need quite large, relatively flat, easily accessible and interconnected areas, so a cliff side city is probably unlikely. At the end of the day a blast furnace is a blast furnace, is a blast furnace.
Socio-economics
As mentioned elsewhere, it’s quite likely that damaging the ability of an individual to fly would probably be used as a severe punishment. One interesting thing to speculate about is how can avians be effectively coerced? Victorian industrialisation was a brutal, foul and miserable experience for the working class, prompting sabotage, strikes, and violence. If workers had the ability to fly far away quickly on a whim, would they suffer such harsh treatment?
Perhaps there is an opening to imagine avian society as ring based on anarchist or consensus politics. Alternatively, perhaps the majority of the population are kept disciplined with forced clippings or pluckings for an airborne capitalist class living palacial towers.
