Would building cities as giant buildings be a practical defense against flyers?

[Psionic Dog]

For a city wide defense I'd image the best the humans could do would be to string a mesh canopy from building to building to cover the streets. Nothing substantial, just enough to block spell Line of Effect, provide citizens partial concealment from the skies, and to give sentries a chance to notice invisible flying scouts trying to cut their way in.

[Lvl 2 Expert]

I disagree with people here that the building needs to be completely self sufficient. The power of flyers is that they can attack anywhere at any time and you can't guard every place all the time. But you can guard all your convoys with supplies as long as the city itself is safe.

The city will need to be able to stockpile supplies against prolonged sieges, but that's true for real world forts as well. It's merely made a harder to do because with the whole top side of your city to secure in addition to the sides you'll want the place to be as small as possible.

For storage I'd therefore recomment basements. Maybe a nice roomy cave system. Natural caves would be nice, marl caves or a salt mine or something of the sort might be even better. Flat floors, straight walls, kept dry. Dug for economic reasons, with a secondary military use. Any large underground structures increase your vulnerability to diggers and sappers (caves leading into a mountain side are more secure), but what flyer would voluntarily give up their mobility bonus to risk fighting in a cramped tunnel system?

The roof/canopy itself is still a huge engineering challenge. An alternative might be an anti-air weapon system. Assuming no cover for the flyers and little armor they would be vulnerable to return fire. And the system doesn't need to be mobile. But it does need to be either very accurate or have some sort of area of effect thing going on, like the explosive munitions in flak cannons or the stream of bullets comming from machine guns. It also prefetably needs a pretty massive range. Not just to keep the enemy further out, but to prevent the ammunition from raining death and destruction on your own city. It's not as good as a solid roof, and still a huge engineering challenge, just maybe not as huge a challenge as a roof would be. Even historical castles had prettu flimsy roofs.

And let's not forget roofs have their own vulnerabilities. Fort Eben-Emael was the crown jewel in Belgium's main line of defense against the German assault at the start of WW2. The thick roof was imperveous to any bomb known to man. But it made a fine landing platform for airborne troopers which quickly conquered the fort. Against full on flying people just a roof with no additional protection is an invitation to come chill at your defenses and take their time finding the best way in.

Psionic Dog may be onto something with just the mesh/cloth canopy covering the street. It means the enemy has to come close to do properly targeted damage, so it gives active defenders and defense systems a chance. It even protects against "what goes up must come down" friendly fire somewhat.