I am in construction of a city that began as a dam. It took 2 weeks to build the damn and after a month of ignoring it i decided to build a city around it. It is near complete with about 6 or 7 skyscrapers, houses, roads, a wall surrounding the city, power plant, fallout shelter, and tnt cannons surrounding some of the sides. Now I just need to add lighting inside to make it mob spawn proof
My most recent massive project (well not really all that massive, but large scale) was the building of an arch style bridge across a span of water 32 blocks wide near a snow biome SE of my castle. Images of the bridge at daytime and nighttime are in the current Beta Survival pictures thread.
I made it entirely out of bricks, and used over 700 of them to complete it. That was all but 43 of them. I've pretty much depleted all the clay within a day or two boat trip from my house.
You can check out my big castle/city for some ideas in the video below. It's large, easily expandable, and has 4 zones. Keep watching for some stuff that my friends have done too, they've done some awesome stuff.