Well, I had no plan of having no wall anywhere. Actual rooms (dining, private quarters &c will have a wall and doors). I was talking solely about the workshops and their stockpiles.
This bazaar / plaza / forum / market will be deep in the mountain, behind several drawbridges and plenty of traps. Actually, my river defense will consist solely of traps -- the military, if I ever have one, will be stationed more towards the magma. Or maybe not, but at any rate the archery targets will be near the magma.
I can seal off the river, but towards the magma there will be a wide open space and several metalworkers and the likes. I also think about deploying "decoy dogs" on ropes next to the magma, hoping that they'll catch the first fireball.
Coming to think of it... will a second and third line of defense be any good at all? I never experienced a siege myself, but from what I gather it's difficult enough to get the dwarves into the fortress. Gathering them behind a second line can't be any easier, besides, if the first moat and drawbridge didn't stop an invader I don't think the second would.
Edit: as I have no other place to put up screenshots, the current plan is up on the Wiki. The basic idea is to knock out the remaining walls between the seperate working areas.
[ March 13, 2007: Message edited by: schnobs ]