Can Z Brush also Create Architecture? Perfectly straight, rectangular buildings and such?
Yes, of course. Even zspheres can produce angular structures, since they are actually cubes. No different than box modeling in a 3D Animation package. Zbrush can also import pre-built structures in DXF or OBJ formats as well. If you are an architect you would want to use something more tailored to suit. But if you are an artist, then Zbrush will work just fine. It all depends on whether you want to be technical or artistic. Zbrush is tailor made for the artistic temperament.
I use Zbrush alongside Cinema 4D, thus giving me all the options without all of the hassles, (of, say, Maya or Max which have steep learning curves and more technical interfaces).
Zbrush has all the basic architectural forms, just like any 3D program, (block, cone, sphere, cylinder, etc.). booleans, displacements, repeating shapes, all the things you need to model and paint a building or structure.
Check out some of the work done in the ‘‘Featured Gallery’’ section (top row).
…has not yet “Warm points” (snap points between object) and not unity size system inside it’s some difficult to align objects(without some tricky scripts) to the precision needing in architecture 
But you can make crazzy architectures with it
with incredible level of details 
Have happy Zbrush architectures
PS If you want “straight edges” on any volumes just disable Smooth H, V in Menu Tool/ Display properties / Smooth Horizontal, Smooth Vertical
Bevel H & V can also be regulated
For some volumes you must also decrease Numbers of Hdivide, Vdivide in Menu Tool/ Initialize