For my next RPG campaign I want to make a city on the back of an outsized sea turtle. It will be a trade port with an older very run down section and a newer fancier part from renewed commerce. I've not pinned down what style I want to try to execute the map in, aside from that it will be fairly top down not isometric as the size of the city and my desire to track movements on a scale down to house-to-house make isometric unwieldy. I've only gotten some base details of what I want for the world and would invite any of the many world builders in the guild to toss in ideas if they like.

The Map: The turtle in question has a back of which the none submerged section is several miles across in either direction, unlike an actual sea turtle it has a more ridged back (like that of a snapping turtle) that form a series of oddly regular hills. The turtles head is to be avoided so there will be a light house there, and the tail section is prone to irregular currents from the motion of the turtle swimming. Said turtle doesn't have much vertical motion in the water. The city is large, makes most of its wealth through trade, and supports itself through a combination of trade and fishing (the turtle seeks out the richest fishing grounds making harvests well above average). The city is newly popular after having been in long decay so new sections are springing up among the derelict sections. I could use some suggestions on what patterns of urban decay and renewal are most likely as I have no idea at all about such things. I've tried searching for some good examples of representing terrain height in a city, but aside from using topography lines I didn't find many. This might just be due to my poor seach skills.

TLDR RPG related material: The world will have embodied animal gods, different gods like to throw around their divinity differently. In the case of turtles its thrown out a number of truly outsized specimens (because when you are a turtle the best thing you can be in bigger and tougher than anything else, why mess around with anything thoughtful or clever?) that have become competing merchant cities (they sink way less often than actual ships and can carry a huge amount of goods). This gods are by no means immortal, but the divinity gets recycled when they are destroyed. I'm considering one of the other turtle cities has recently died off opening a better territory for the turtle that our headline city lies on thus the reason for its changing fortune. I want a number of places for the party to explore that they will have little or no knowledge of, not too much time spent getting to these unknown lands, a good urban sprawl environment for city adventures, and good stable of familiar NPCs to continually interact with. A moving city seems a good solution to all these problems. Magic will be physical, animal based, and limited to a smaller section of the population. The lack of fireballs and ice-storms will make magic less of an army breaker in the direct death sort of way, but leave it powerful information gathering wise. That's really all I've worked out so far so any brainstorming type ideas are welcome.