Part of me doesn't want a map because it feels each detail cuts off a multitude of possibilities (thus the abstract city of Nidus). But I'm at the point where it isn't really fair for players, now that there are several locations they've encountered, to not let them know how places are situated in relationship to each other. I could have drawn my own map but the old simulationist in me starts whispering: "But what of the geology? The scale?" and I start researching island geology and I end up spending hours and still have no map. So hell, I'll just use a real map and give myself freedom to fudge any details on it that don't work toward my ends.
In tracking pictures that were sources of inspiration for Nidus, one place I ended up was this old map of Santorin Isalnd that struck me as just the right style. I was ignorant at the time that it was the quite interesting ancient island of Thera. Here is the public domain map from 1848:
Here is a version where I've removed most of the place names and some details to allow for customization:
I always imagined Nidus as a cliffside city facing the open ocean, but here the water inside the ancient caldera is the much safer harbor, so I'll probably say Nidus is located where Thera actually is, although it's bigger and climbs higher.
One thing cool about using a real location is that you can find actual satellite pictures. So, if the party finds a magic carpet or tames a flying beasty I can show them this:
Of course, I'd probably want to edit out that airstrip first. :)