Well continental Asian (particularly Chinese and Korean) influence goes back to as early as the Yayoi Era (400-300 BCE - 250CE) as it was mentioned in the Chinese
The Book of Later Han in 57CE and Japan even sent tributes to China during the Kofun Era. In fact Japan didn't even become an independent state until the Nara Era.
Now by 1543 which was the Sengoku Era (the era that the Feudal half of
Inuyasha takes place)Europe (specifically the Dutch, Portuguese, and Spanish)had began establishing trade after a Dutch ship was blown off course and happened across Japan. (Of course in the Edo Period, the Tokugawa Shogunate, kicked all foreign powers back out.) This was also when Christianity was first introduced to Japan, but I digress.
Therefore, it could quite easily be taken as creative license and potentially FACTUAL that there could be European influence in a Japanese shiro (castle). Especially if said daimyo (feudal lord) was affluent in trades.
And like a good writer, source time! A quick overview of
the History of Japan .
And in the name of informative eye candy
Japanese Castles !
Hope this helps clear the air on some canon vs. fanon issues.