I sort of assume that someone else has previously discovered this, but it's one of the more beautiful and useful of my recent experiments in fortess design so I'm going to post it anyway. It's a 5x5 4z level ramp structure that has four independent spiraling ramps. The ramps don't use diagonals and are easily walled off for maximum minecart use.
= : Anything. Wall, floor, stairs, empty space, etc.
These components should be vertically stacked like this
_ : Empty space/downward ramp/channeling designation
^ : Upward ramp
X : Wall
_ : then a ^, X,_,^,X,_ and so on.
X^_=X z = 0
====^
_===_
^====
X=_^X
=X^_= z = -1
_===X
^===^
X===_
=_^X=
_=X^_ z = -2
^====
X===X
====^
_^X=_
^_=X^ z = -3
X===_
=====
_===X
^X=_^
and then back to the beginning. If you want to get silly (you do), you can also add a double helix ramp structure inside it.
X^_=X z = 0
=_^X^
_===_
^X^_=
X=_^X
=X^_= z = -1
_^X=X
^_=_^
X=X^_
=_^X=
_=X^_ z = -2
^X=_=
X^=^X
=_=X^
_^X=_
^_=X^ z = -3
X=_^_
=X=X=
_^_=X
^X=_^
...and still have room for an up/down stair in the middle.
The general idea can be scaled up pretty easily. My math is rusty and suspect, but I think in a X by Y figure with N sides, the (p)erimiter in terms of DF tiles is going to be
[2x + (n-2)(y-1) - 2] = p (works with hexagons and squares at least)
and the number of ramps you can fit around the edge in this way is (p/4). Or just x-1 for a square. Yay math is fun!
Anyway, so that means you could surround this with a 7x7 square with 6 ramps, then a 9x9 with 8 ramps and so on.