Probably your best source is Matt Riggsby's
GURPS Architecture which is still available at
www.mindspring.com/wombat3. [wombat3 is Matt Riggsby.] GURPS Architecture does not presume any use of magic in construction, though you could probably use magic. GURPS Architecture covers more than just Low Tech architecture and more than just castles but it will get the job done. If you have to import some materials because they aren't natively available, you may want to look at his
Silk and Spices article from
Pyramid Vol. 1 #26, which is also available on his website, as it deals with transportation costs among other things.
By the by, nice to see your updated campaign history.