So I realize I do not give good descriptions for locations, regardless I'm going to try and give some in depth descriptions for Griffonstone in this AU.
Now first the city itself took inspiration from Lord of the Rings, to be precise the City of Minas Tirith for it's ascent up the mountain. Unlike the a fore mentioned city this rendition of Griffonstone would be much bigger encompassing the entire mountain. Now that would make it bigger then Canterlot yes but it is the only major population center of the Griffons, unlike the Ponies who have a few such as Manehattan, Las Pegasus, Baltimare, and Vanhoover. I will mention other towns but their sizes vary never reaching the size that would allow them to be considered a city by any standards.
Now as far as my reference to Minas Tirith the way it was built up the mountain is as far as I went. For Griffon architecture I envision something similar to Eastern European Architecture of the medieval eras. Mainly leaning towards Germanic architecture. Fun Fact: Griffons still use candle lighting, or oil lanterns.
The City is divided into districts (not rings as a district may encompass many rings) from the top to bottom there are four. The Palace at the top, below that is the barracks district which also houses the airship dock for warships (dubbed Zeppelins), below that is the residential district (it has stores spread around it to make food easier to access, finally below that the few final rings and the ground around the mountain is the economic district which houses many of the various services the griffons use along with another airship dock this one for citizen use (transport, cargo all that.)
Defenses
Griffons are a military savvy race that had to adapt to a lack of magic, thus as such technology was a focus. Their Airships are armed with weapons surpassing cannons, what exactly?
Your hint is that I play too much Guns of Icarus.
On to Griffonstone's defenses.
However via ground assault Griffonstone would be a difficult task. all the way up the rings would be a bottleneck that would be an easy defense. Not to mention Griffons rule the sky through Talon and Airships.
But what about a siege.
Well perhaps I will come back to just how Griffons would defend Griffonstone from a prolonged siege later but for now I won't spoil any more.