You know, that just reminded me... The opening scene of
Carnival Row shows military tactics to control low-level flying fairies:
As I recall, there was at least one other battle depicted in the series, though that one seemed pretty iffy, even to someone like me who's completely lacking any tactical sense whatsoever.
The presence of the anti-flying netting in the trees suggests it's quite intentional that the soldiers have driven the fairies into this area. It looks more like a hunt or extermination than anything about war. Why they'd ever be on the ground in the first place is beyond me.
In the real world, aviation was a massive game changer in war. In a fantasy setting easy access to flying would be similar. One fairy or aarakocra with a bag of holding on each hip would be the equivalent to a stealth bomber...or a mass driver.
A 2nd-level artificer can infuse two items and can replicate a bag of holding. "This bag has an interior space considerably larger than its outside dimensions, roughly 2 feet in diameter at the mouth and 4 feet deep. The bag can hold up to 500 pounds, not exceeding a volume of 64 cubic feet. The bag weighs 15 pounds, regardless of its contents."
So just craft a stone cannonball less than 2 feet in diameter (so it can fit into the bag) that weighs slightly less than 500 pounds (so it can fit into the bag). A round ball of sandstone that's 1.87ft diameter weighs 496.62 pounds. For granite it would be a ball 1.77 feet in diameter weighing 498.5 pounds. How long would it take that ball to reach terminal velocity? Any takers on how much more damage it would do than a cannon, catapult, or trebuchet? It's effectively a mass driver.
Altitude would be great. Up to around 5-6000 feet you're still good to breathe. If not, there's spells and magic items for that. Plenty of room for that terminal velocity thing, right? Nothing could reach you from the ground, so any fortification worth protecting would have to have flying protection. But they're not going to be able to patrol the entire 5-6000 feet above a castle or city. Say a mile across up to 5-6000 feet...that's about 131,373,864,951 cubic feet to cover. I'm sure one or two dudes on griffons with crossbows would manage fine.
Fly over the target and dump out the bag. Level a castle. Find another target and dump out the second bag. So a second level flying character with a few weeks to spare can level a castle. Hell, it's practically a downtime activity.
But nope. Flying is nothing.
(Yes, I know how absurd this is. That's the point. I have had players who would do this. It's one reason why I can't play Spelljammer. They'd just use it to take over the world. But hey, now they don't need the Spelljammer. Just one fairy and a bag of holding.)