Dwarf Fortress Build Ramp / Stairwell Down After Having Removed Slope

I tried to build an uncrossable pit and removed all the slopes going up out of it. But now I need to cross that uncrossable pit. Is there a way to build a path down from above?

Here's the top layer:

And the layer below it:

Solution 1:

Using the build construction menu (b > C), you can build ramps and stairs. You can build a down stair on the level where your dwarves can access, build up/down stairs below each other until you reach the bottom, then an up stair at the floor.

Simple diagram viewed from the side:

---    D_P__    -- P = Your dwarves
   |   X|       -- D = Down stairs (b > C > j)
   |   X|       -- X = Up/Down Stair (b > C > i)
   |___U|       -- U = Up Stair (b > C > u)

Solution 2:

Construct a raising or retracting bridge over the pit. Then you can connect it to a lever and your pit remains uncrossable when the bridge isn't down. This way you don't have to deconstruct the ramps or stairs later.

Added bonus: you now have a handy death trap to fling or trap invaders.

Solution 3:

My guess is: the stones selected for building are the stones in the pit. But they cannot be reached without the staircase! Try selecting stone blocks, as you know there are none in the pit. Or wood. Or metal. Or whatever you want, even clay would work, but not stone.