I always envisioned Tharwain as something distinctly high feudal. The age of massive castles, tight cities with tiny roadways, and farmland that stretched far around the city.

There's a little planning around the main roads, fortifications, and plazas, but most of the city is a product of a natural progression outward. The walls form a sort of limit, but beyond them are small communities and farms. Here's another image, but it's a little big so I'll leave it as a link:
http://img237.imageshack.us/img237/1765/stadtplanferdokxg2.jpgCaeluin and Varnost are hard to search for, since by their concepts they don't build for grandeur. Caeluin builds for practicality, and Varnost for expediency. This necessitates simple materials. It doesn't mean they lack style. For Varnost, the idea would be almost like a temporary camp more than any permanent structure. I think distinguishing between a military structure or camp that was built for a purpose and a jury-rigged structure built as a temporary abode and to give each its distinctive style is important, but I'm uncertain as to how to represent each. That's difficult to image search for, since such structures rarely endure the test of time. Caeluin from its conception was supposed to be a fusion of peoples and cultures. In that sense, their architecture should reflect this, but I'm unsure of how.
Now, Balkurn and Nisyrra are where things can get creative. These are factions that take
pride in their architecture. I think Nisyrra would be the best to pursue an Islamic or Indian style with. I originally envisioned them somewhat greco-roman, but I never felt it fit perfectly. I think these styles of architecture maintain the aesthetic flow that I envisioned, but pursues a style distinct from the other factions. This seems to be the right direction for Nisyrra.
As for Balkurn, all I can say is that you can't google image search the word "fortress" without seeing something that represents them well. They build big, grand structures that dominate the landscape, and use it to their advantage. Their entire city is one extended structure, and I think one thing which I won't be able to find anywhere is their tendency to build in "layers", using networks of archways and walls to connect their settlement almost like a maze. This is the closest I could find to representing that:

Also see:
http://www.edwebproject.org/oman-dubai/pics/nakhal.fort.jpghttp://k43.pbase.com/u25/cjo/large/15156845.fortress.jpg