How do I build above-ground fortress features?

Question

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?

Answer 1

Well, it's kind of tough. First, meditate upon the Constructions page on the magmawiki. This may explain some other problems you'll encounter soon.

Your specific problem is caused by having no stairs on the second level of your tower. Remember that ordinary dug stairs only work if you have a "down stair" directly above an "up stair". Constructed stairs work the same way. The only square your dwarves can reach is the one directly above the stairway. In this square, on the second level, you need to b-Construct a down stairway or an xup-down stairway. After this your dwarves will be able to get to the top of the first-level walls (which come with built-in floors above them; do not build more floors on top of them because it will prevent you building walls instead). I recommend building a floor on the interior of your tower before extending the walls upward, and remember to build the corners first.

Answer 2

Your problem is that you are trying to build in an inaccessible location, a dwarf must be able to stand on the same z-level as the structure being constructed, in a horizontally or vertically adjacent tile.

Legend: C - construction, A - able to build, U - unable to build
UAU
ACA
UAU

You can build rafters to let dwarves get up there (An adjacent "wall" of up/down staircases works particularly well for this). In your particular situation, you need to build a down or up/down staircase to let dwarves up.