Excellent idea! It makes me think of a fantasy book with cities close to your concept (it wasn't turtles but mythical beasts). in this book, the city and its inhabitants is a kind of a symbiot with the giant beast : they free it of its parasits and problems, took resources from its waste (like used scales for building, etc.).
Do you intend to consider the turtle as unique? I mean, other turtles with the same size would pose possibilites or problems for the city.
OK sketches I'll get on that. The turtle is not unique, there are several of them. Each has a standard route that represents a major ocean basin such as the pacific rim, the Mediterranean, or for a young turtle god the gulf of Mexico. The towns on them compete mainly through trade. The turtles themselves avoid each other rather than going to pathetic turtle battle. I was thinking the death of another one has altered this ones route to new lands less known by the inhabitants of this turtle, thus new possibilities for trade and as its for and RPG ultimately adventure. Who knows what other new and exciting animal gods they can't meet along the way.
Defense of the town will be a real issue, it not only moves but lingers at times (both to feed itself and allow for adventures without your down moving on without you) and its route is predictable enough to allow trade meaning anyone who doesn't like you has time to prepare for your arrival and only needs a fleet of small boats to get to the city as it comes close to shore. The turtle isn't in much danger a 60' wooden spike at the size I've laid out is still shorter than a thumb tack as far as trying to ram the turtle and its made of tough stuff (though not as tough as it would have to be for the weight of its own skin not to pull off its bones if real physics applied but clearly we are ignoring those to allow for a several mile long turtle you can't make me think of the cardiovascular nightmare scaling up a creature this large would lead to). As a off idea for defense I was considering maybe having mirrors shine light on the water the sparkle catches the turtles eye and he thinks there is fish there and turns in that direction. Its not going to make him steerable in the long run just get him to face an enemy, but that might be overly gimmicky. Most ancient navies where extensions of merchant fleets (Venice is a great example of that) and as the turtle is accompanied by a large fleet at all times to move goods to and from shore and fish the navy idea seems a natural extension of this.
Looks like using just run off with cisterns to collect it a city of good size could be supplied, which makes sense as that's all that is happening on land in the long run. I'm using Ventotene as a rough estimate (just because I remembered the story of the exiled Roman princess and that they had to collect rain water there). The estimates of water use per person per day in medieval society seem to fall into the 3-5 gallons per day range. As the shell is impervious to water that leaves a good sized collection basin, not sure if anyone is better with these number than me, but I'm going to assume the turtle travels in fairly wet conditions as coasts tend to be giving ample opportunity for rainfall to collect and making building for rainy conditions a necessity.
I considered making the shell a resource too, one that has to be carefully used as you don't want to give the turtle an itch by cutting away too much. It would be a super hard material a bit hard to cut but great for armor pieces and the like being divine turtle shell. Not sure if that's adding to much economic factors in that I wouldn't have to track and consider later on.
Probably going to make the turtle in turtle fashion dislike others of its own kind, though I think the idea of a flotilla of baby turtles is adorable and you should make that right away so I can see how your turtle city turns out so I don't feel bad about not using that awesome idea.
Edit: If I start liking all the great ideas and points people are throwing out I think I'd have to like every post here. You people are great.
Sketch to give some idea of what I'm talking about, might be totally unrelated to the final map (still can't decide what style I want to do) but at least shows my thought process (for some values of the word thought).
Looking good! I like the idea of using the shell as a resource, assuming it could be cut properly surely it'd be an ideal material for ship-building?
Ok I've done more layout work, but I'm having a hard time finding out exactly how many people could be supported on a given area of water collection but the crude estimates I have based on an average rainfall of around 60 inches a year (the turtle tends to visit wet coastal areas) and a person requiring about 5 gallons of water a day (the high end for medieval) comes out to about requiring 1825 gallons a year per person which takes roughly 60 square feet of surface area at the rainfall I've listed (assuming 100% efficiency which isn't going to happen). Assuming a probably over generous 50% efficiency (so 120 square feet per person per year) a square mile could support 232k people (this number does nothing to account for agriculture just personal use. That looks wrong to me where have I messed up or is it just my unrealistic efficiency throwing the number into question? Or is the simple answer that there is more than enough water to support a reasonable sized population on my turtle even assuming poor efficiency of water collection and storage. By comparison the couple square miles of farmable turtle produce only enough food for a bit over a thousand people (using the classic ~2 acres per person medieval farming number), which is fine as any city is supported by the countryside around it, it just happens that the countryside on the turtle is a moving one. I was thinking of a population in the 20k-40k range, does this seem reasonable for a large medieval trade city with only a few miles of surrounding land?
On the map side of things I've decided on a more "painted" style which I've never done before but should be fun to try my hand it. Trying to learn from my past errors I'm trying to work out my layout before hand. I've laid out areas for insets, borders, and the like to begin with rather than adding them at the end. the lines on the back will go away replaced with much lighter sketchier lining but they are there for ridge and gulley guides for me while I try out the fancy stuff. Its off center because the port side of the turtle is more developed and thus a focal point, but I'm not set on the idea if others think that is madness.
It all depends what "5 gallons of water a day" constitutes. I'm guessing it'll be the water used for drinking, cooking, washing, etc - everyday water use. In that case, the reason the numbers this gives you seem extremely low is that this particular section of water use is actually a tiny proportion of the amount a person's daily life consumes.
Take food for example, it takes over 100 gallons to produce 1 pound of corn, about 100 gallons to produce a single potato, and exponentially more to produce meat, such as 4,000 - 18,000 gallons to produce a hamburger. Clothes also use a lot of water, a modern cotton t-shirt taking 2,700 gallons being a common statistic. Industry also consumes a huge amount of water. Generally speaking, water use embedded in property and food seems to outstrip "basic" water use by a factor of several thousand (or actually tens of thousands if you're eating a lot of meat and consuming a lot of processed goods).
I guess that's why it's hard to find out concrete numbers - so much depends on the level and distribution of technology in the society. A less developed society would use a lot less water than one with heavy industry. I'd guess that the population density estimates people generally use would have this kind of limitation included inherently - at medieval technology levels, those densities would make sense based on all limitations, rather than just crop productivity. Also, a heavy dependency on fish as a source of food would help to lower water use. Maybe they could also plant and grow kelp underwater too (might have been mentioned before, but I can't remember).
As to the map, looks like a good start, though it bugs me that the shell isn't centered! :D
I also think it'd be nice to include the head in the map too, it looked better when it was included. :)
Thanks that helps clarify my thinking a bit. I'm less worried about food production than just everyday water use. Water is simply too heavy, and high in volume to picture a city thriving if it has to make constant trips too and from the shore to supply it with water. Most large medieval cities probably produced only a tiny fraction of the food they consumed within a couple of miles around the city proper and I don't figure this one should be any different. Being mobile will help insulate them from the worst of famine. I do like the idea of developing the cuisine of the city a bit as well as some possible specialty goods (purple cloth anyone). Seaweed, shellfish, and other things that grow well on the shell could be the old peasants cuisine for the area. Being a large city it would ideally need enough water to fuel at least a medieval level of processed goods, though its role as a goods delivery system would make it just as profitable even if it didn't produce much anything itself.
The "turtles views" on the down are an excellent idea!
Very cool idea, indeed.
Do you happen to know the Dino Riders? (https://en.wikipedia.org/wiki/Dino_riders - Picture: http://www.allmystery.de/i/tkavDIg_dino20riders.jpg)
The "good guys" in this series used some Kind of telepathy to communicate the dinos and to have them follow their will.
You might want to introduce something similar in your city: Each turtlecity hosts 1 mage whose job it is to keep the turtle and the damage it might cause under control. Maybe he can not directly control the direction the turtle swims, but he can install, operate and maintain his magical devices protecting the city from the turtle's shore leaves or it's dives. Or he uses his magic in order to bait the turtle into a certain direction. And since he is a scholar, it is also his job to map the journeys of the turtle and everything encountered and discovered on the way.
Regarding your city size: As you noted, you did not account for agriculture. Neither did you do so for ather things such as cleaning, steam-power (or rather water-power?) and the likes.
I come from Ladeanburg (https://en.wikipedia.org/wiki/Ladenburg), a town founded about 2000 years ago. Under the rulership of the Romans, it used to have a trade harbour, a marketplace, a library, a theatre, a basilika, a forum - buildings only Major cities used to have. Population: a couple of thousands. Even today its population is ~12k (and the political importance of the town has vanished over the centuries).
So: 20k-40k rather represents a vast thriving metropolis.
One question you should ask yourself: A city that is most of the time on the water will have only very few people coming there from elsewhere with the intention to stay there. Since the City has not been "planned" (as in "hey, let's send a couple of thousands colonists there to raise a giant city on a giant turtle") but started out as a small fisher village, then why or how did the population reproduce, if your reply is NOT incest?
Here's a tip from me: try not to get bogged down in plausibility details like whether there is enough food or water for a certain population. You aren't on trial, and so 'beyond reasonable doubt' isn't necessary. Definitely, I think, give a nod towards those things--add as much farmland as possible, in a way that looks convincingly like it could have been done artificially or naturally--but trying to work out precisely how much is just going to cause you a headache (and nobody is ever going to try and work it out anyway). Consequently, I think it is more important how the layout of the farms appears, than their number (i.e. are they laid out in large fields, with communal workforces, or narrow plots that are divided amongst the peasant population, and so forth). Each to his/her own, of course.
So, looking at the layout you've added so far, I have a few questions that you'll hopefully find helpful:
--the town starts near the tail, and then grows headwards: why? is there some advantage in the city moving towards the head rather than the tail? what were their priorities when the town expanded (eg. more room for fishing jetties, closer to the fields, away from polluted water etc.)?
--as Schattentanz commented, how did the population grow and change over time? Natural growth (births minus deaths) tends to be too slow to result in substantial population growth; your city would have needed to be founded many thousands of years ago if it was only natural growth. By contrast, immigration is, as stated above, less than easy given the unusual nature of the town. If I might venture a suggestion, perhaps the settlers of this turtle were refugees (i.e. did not have the luxury of choosing a mainland place to live, migrating in large numbers over a short space of time, bringing with them few possessions but an entrepreneurial spirit etc.).
--same question as above, but applied to the "upstart estates" and new town.
--where is the most prized 'land'? is it near the water, or the higher 'ground' of the shell? do they prefer to be close to the head or the tail? where is it the most stable (if this turtle was going onto land, the front is going to face major upheavals, as it hauls itself onto a beach, but the back might have sand flung over it as the turtle moves--just watch one of David Attenborough's nature documentaries, and you'll be able to see how awkwardly turtles move on land).
--I know you mentioned that the inhabitants of the turtle brought their traditional style of architecture with them, but I always find it enjoyable and rewarding to try and think of what adaptations a population has made considering their position. How has this population adapted to life on a turtle's back? What can they do now, that they couldn't before (and, equally, what can they no longer do, which they used to do often)? How has their architecture changed (for example, being resistant to earthquake like tremors)? How has their calendar changed (not staying in the same place surely means they have to think differently to landlubbers about planting and harvesting times)? Have they added/dropped any festivals from their year, and do they have special locations to hold festivals/markets/public events? If space is at a premium, I think it is likely they will try to give a place multiple uses (eg. the stadium doubles as a meeting hall; the church is also used as a schoolroom; the town square serves as market, meeting place, and festival ground etc.)
To sum it up more concisely, it is a ridiculous idea (I think) that living on a turtle is not going to require some level of lifestyle/cultural/economic/architectural/religious adaptation.
And regarding Schattentanz' other comments, a) I don't think magic is a good idea to explain how they navigate the turtle (if they do at all); I think you suggested at one point that they shine light on the water to suggest a shoal of fish, that the turtle then heads towards--that is, I think, a really clever idea, and gets away from the magical fix-all (no offence Schattentanz); b) for a good idea of the kind of services/goods available in cities of various sizes, this fantastic tutorial/guide should be used (I once tried to check it out to see if the stats really added up, and as far as I could tell, they do).
I'd like to see some more sketches, when you have them. It's looking good (the inset views in turtle-frames are an excellent idea).