Dwarf Fortress Build Ramp / Stairwell Down After Having Removed Slope

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)

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.

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.

One thing you might consider is to create an access door to the bottom of the pit. Even though it might feel more secure to have a pit with no way out, often you will need access to it. One reason among others is to reclaim crossbow bolts you used as well as goblinite from fallen foes in the pit.

1) From inside your fort (or from outside if you are paranoid), dig a up/down stairway down to the level of the bottom of the pit.

2) dig a horizontal (on that Z level) tunnel that connects the stairway to the bottom of the pit.

3) build a small (1x1) drawbridge where the tunnel reaches the pit as a doorway that allows you to seal the pit.

4) connect the drawbridge to a lever located at a safe location in your fort.

The end result: You now have a doorway that the majority of foes cannot break. The outside of a raised drawbridge will act like a constructed wall rather than a door; building destroyers who can beak down normal doors cannot break closed drawbridges. If you ever want your dwarves to enter or exit the pit you can now pull the lever in your fort to open (or close) the pit access tunnel.