Build a fortress out of a colossal 3x3x3 Rubik's cube with a minimum of 3 tiles per square (x and y, and 3Z tall) (including Z-axis (3x3x3 individually per lesser cube that makes up the whole (minimum: 9x9x9 or 11x11x11 (with spacing)), and a minimum of a 1-tile/Z thick gap between cubes (either empty space or some other material than the colored ones used. Wood is acceptable, I guess.).
Easiest setup: All sides 1 color each.
Hardest setup: Use a scrambled setup. (use your own cube as reference)
2x2x2 is more challenging by compression, and 4x4x4 and 5x5x5 may get too easy; although 4x4x4 has no wildcard cube, like it's 2x2x2 counterpart (applies to all even-dimension variations), though you still need to make them with more materials with matching color for the squares on each side. Bottom floor would be floor tiles, of course. Each block is specialized by color code.
Center cube (2x2x2 coordinates) is either transit or a wildcard, seeing as there's no color associated with it (and being the main axle; which also explains why 5x5x5 can get too easy, unless the inner 3x3x3 area follows similar properties as the original challenge).
In ROYGBIV order (though placement of the sides is your choice)
Red = Military
Orange = Civilian
Yellow = Stockpiling
Green = Workshops
Blue = Hospital or Water Supply (I would maybe advise this to be the floor, but provided you can find enough blue material to make it all. Then again, I guess anything else that can be associated with Blue can work.)
White = Nobility
Corners will be multi-classed by 3 colors, edge sides will be dual-purposed by 2 colors, and center-sides will be single-purpose by color. This will be one heck of an architectural challenge, especially since you'll have to plan internally to make it all functional and mostly self-sufficient for a minimum of 5 years. Above ground for all to see, and suspended at least 3Z from ground level as well.
I like that I came up with this, it sounds pretty fun.
EDIT:
As long as all sides would be visible (especially through a visualizer; for proof), and it is suspended a minimum of 3Z, then we're good.