I try to keep the above-ground sections of my fortress limited to just a few necessary functions but I always felt like I was a little unprepared when goblin attacks started flowing in. I could wall myself off but I would rather just give myself an absurd advantage over various maladies that visit.
I wanted to make a small vertical fortress entrance surrounded by a ground-level wall and bridge and then a second level of fortifications accessed by an up stair to a level (or at least a ring) of artificial floors. My archers can then just patrol the fortifications and kill anything in sight without risk of painful punches.
Floor 1 (just a ring of walls with one ^ up stair):
OOOO O^ O O O OOOO
I'm having trouble building floors (or walls) on my second level though. All the second level squares are either red or purple X characters (purple being "No access to building material non economic item" which doesn't make sense because I can build more walls on the ground floor).
How can I build walls and floors above this level?