Fantasy cities are fun. Because they're never really normal places. They're accumulations of potential backgrounds for scenes, tacked together with an overarching feel and maybe a lot of oblongs drawn on a map (I used to do this, before I realised why it was pointless)
For me though, it's the feel that's the important bit. All my favourite cities in fiction generally, not just in fantasy, aren't about down to the last detail descriptions of the place. Guidebooks don't manage that, after all, so a novel certainly doesn't have time.
The best bit is that you get to create your own city out of even a real one, just by the feel you give it. It's the same way that painters could paint the same place, and still produce different works. My take on York is probably nobody else's, but it's the right place for a couple of my novels.
I particularly love places that show up in fragmentary phrases, just in passing, and you know it would be cool to set something there some day.
Take this passage from a MG/YA fantasy novel I've got laying around:
Varansburg, in Illthria’s Border Marches, is not, by any stretch of the imagination, the most important city in the known multiverse. Or one of the top couple of hundred, for that matter. It will never, for example, match the bustling industry of the great ant cities of Hive. Nor will it come close to the mind boggling oddness of the city of Corner, where other dimensions impinge almost at random, and where it is frankly impossible to nip down to the shops for a pint of milk without wandering through a couple of spare worlds, a trackless desert, and the storerooms of the British Museum. As for keeping up with the sultry delights of the cities of the Isth peninsula…
Okay, maybe not my most wonderful writing, but I knew as soon as I wrote those places that just from those simple ideas, there would be something fun there. I might even revisit them at some point.