Heights would not be dangerous
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.
Stairs and lifts would not be necessary
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.
Vertical and overhanging surfaces would be practical, everything is 3 dimensional
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.
A room can be exited from any side
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.
Flat space would not be at a premium
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.
Airspace would be valuable
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.
Motion might be acceptable
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.
Weight would matter
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.
Building material would need to be lifted
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.
Dropping an item would be a big deal
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.
Prevailing wind would matter
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.
Fire would be challenging
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.
Industry would be lightweight
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.
Water would need to be lifted
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.
Sewage would need to be dealt with
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.
Wires would be extremely hazardous
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.
Wingspan would matter
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.
Fences would not be effective
A secure structure would need to be completely enclosed. Fences would only be effective against ground based animals. Likewise for a private structure.
Wind would be a constant and reliable source of power
At altitude, wind is likely to be stronger and more reliable. Expect wind turbines to be used for electrical and mechanical power.