If all citizens donate all spare cobble they have to the walls, I doubt that will be a big problem, as normal people often dig basements, and all other underground constitutions will also get give some cobble. Also when the flattening will be done You will also get a ton if cobble.
About the terrain, will the city be built in ordinary forest boom close to the ocean?
I was wondering if I could join the city without q clan, and live at the farmlands place and help with the farms and and if needed some redstone wiring other places in the city.
Also wondering about the animal farm. Will we have q big animal farm with around 100 animals of each specie as it will be a Ron of people in the city, and I assume they won't have any private animal farms.
In the new SMP when it comes, it will be villager trading, so I was wondering if we should have a BIG hotel for villagers that they couldn't get out of the hotel.
And last point fuel for furnaces will be NO problem from 1,3 even in ridiculous high amounts because of that when putting a bucket of lava the bucket no longer disappear
So all that needs to be done is to get lava from nether, 1 inventory ca 30 buckets=3000 blocks melted as far as I can remember.
I probably have a lot of typos and messy writing since I'm on an phone because I'm on vacation, and it's pretty hard to read through what I have wrote.