...on the other hand, with such access to magma all you need is a dump-zone in the right place and you don't even need to atom-smash those mountains of junk.
(I tend to place my Depot as close to the centre as possible[1] with direct-but-blockable access from each of the side-centres wherever possible, perhaps the
raison d'etre of the settlement being at a key crossroads (that didn't exist before I made it that). Only extreme engineering can stop caravans turning up almost wherever they like, even if you can force the wagons on a single-route, so I go for the least mean-travel-time from the start of my trading days, building in 'delaying detours' to my dynamic city-wall systems to accept all visitors (or 'visitors') but lets me make it more
or less time-wasting to do so once I'm aware of them. I've had a whole-map multi-circuit helical/spiral route (with shortcuts at a pull of a lever) set up in times past, though I rarely need to make much more than a quarter-turn near-perimiter (between outer and next-most-inner defences) the default access route, I find. And when the traders arrive, assuming they don't land on a siege, or vice-versa, they quickly get shuffled into the direct axial accessway. But that's just my way.
)
[1] With even-lengthed embark sides and odd-lengthed depot-side, it's usually a compromise choice to make the centre-tile of the depot the 'centre-tile' of my as-symmetrical-as-possible layout! Alternately, I
could set up four depots, rotationally/reflectively symmetrically around the 2x2-tile centre-mark (as far off as in the extreme corners, give or take walls/trenches/etc of control and defence) all with their own internal/external accessibility switches of various kinds...