Ok. Of course, it being DF, I am already running into the need for Dwarven engineering.
Let us say that my desired hallway of traps is at Z level 0. At Z-1 I have channeled a place to dodge into the spears. Z-2 is, of course, an aquifer, and if I dig the channel I won't be able to do the experiment. Z1 is part surface (a depression, like a dry murky pool that never had water, that I was going to use as a surface farm), part planned extra barracks (I'll build a wall and make it a dining room / food and drink stockpile for the troops instead) and part earth. Z2 is all surface (or empty space). After first building a bridge to allow my dwarves access to the depot (see below) and the wall where a barracks was going to be, I'll channel out Z2 enough to make Z1 directly above my trap path all surface. Then I'll also channel out Z1 directly above the trap path. Then I'll build some upward ramps and some elevated floors upon which to build the actual trap path. Then I'll see if I can set a trap on a ramp or if I'll be losing 4 trap squares.
There will be some free space between the spikes, maybe I'll dig it out and put in cage traps to better inspect critters for spike vs. floor !!SCIENCE!!.
My entry way is setup to bring a wagon path past four archer rooms, also in line with 3 ballistae stations and then the entryway goes through the depot, to the trap path (which is itself setup to be surrounded by archer rooms and each path lane in a different ballistae fire, which, ahem, unintentionally ended in the depot, but none of which has yet been dug and likely won't be due to the need for an elevated floor).