While mining out an area for my magma forges my sole miner broke open a wall into an underground cavern and was almost immediatly attacked by Elk Birds. My feeble militia rushed to defend him but having no weapons and no armour they were being easily taken down. With blood quickly filling the corridors and stairwells as the Elk Birds proceeded to slaughter the dwarves in my fort the only sane mason managed to build a wall in one of the corridors trapping most of the dwarves with the Elk Birds but saving some raving mad doctors and children. While the Elk Birds hunted down the trapped and running dwarves I explored the extent of the hole and found not only were some walls open but the roof was accessible as well.
With lots of medication my insane and severly wounded dwarves were brought back into service. I had lost all my nobles but after a couple of years of painstaking labour in cleaning up and crafting I was able to attract more. I trained my military as best I could to an eventual return to the magma forges to claim the bodies of the fallen and what is rightfully mine!
Eventually I broken down the wall in the corridor and stormed the half-built magma forge workshop; stationing my entire army to guard the entrance to the carvens. Diligently miners and masons have worked feverishly to make good repairs and seal off the workshop from the dangers of the underground carven. Maybe, to be explored later.
Now for my question. I've created a walled square with an up-stair, and on the upper level a downstair. Then I built a floor between all the walls to create a ceiling. I then deconstructed the downstair but I can't build a floor from the up-stair below to complete the ceiling. Is there a way to pave over the down stair from the inside so that monsters can't get in?