Thanks for that link feelotraveller, very useful to me. I have been trying to wrap a road, 3 blocks wide, up my tower to no avail. I have one last idea that maybe will work which is to dig a 12 z level deep pit that will actually hold my structure, which should allow me to carve the road around the tower out of stone (avoid (b)-(C)-(r) all together?). I would still build the actual tower inside the pit out of wood, in part to spite the elves and in part because I maybe want to make a system that allows me to not just have a moat around my tower, but actually be able to flood the entire structure inside the pit, and be able to pump the water out again. Sort of a submersible tower. I would create a way for trade wagons to enter from the outside, through a deep channel for them to reach the bottom of the pit, they would make their way up the tower and reach the surface level. At this point my planning stops. I am thinking of using a drawbridge system to create a drop zone where the wagon will fall back into the pit. I plan to create a walled structure the caravan will fall into (our a pumpable "drown tank" maybe), to keep my dorfs safe.
This whole idea of wanting to try and drop a trader's wagon from a 10 z level tall structure and collect the goodies came from me having to learn to protect my own wagon from giant Kea. A while back I had my iron anvil stolen about 1 minute after embark, it was horrifying. So now as soon as dwarven boots hit the dirt, I create a 2 z level deep hole under my wagon, and learned to carefully breach the top layer to make my own wagon fall into the hole, which I then quickly cover.
My apologies, I have strayed off course from my original post a bit. This might become my first mini mega project
p.s. My tower is surrounded by hundreds of cherry trees. My dorfs were hard at work building the great tower, suddenly my screen exploded in the most beautiful array of colors as the cherry trees blossomed. My dorfs all sat down and watched for a good hour. There was drinking, and merry-making