Reddit mentions of Castles of the World Coloring Book (Dover History Coloring Book)

Those do sound like they would fun to color. Medieval seems easier than towns, though here's a city:
https://www.amazon.com/Fantastic-Cities-Coloring-Amazing-Imagined/dp/1452149577
https://www.amazon.com/Fantastic-Structures-Coloring-Buildings-Imagined/dp/145215323X

Medieval:
https://www.amazon.com/Coloring-Medieval-Times-celebrated-illustrator/dp/1600584039

There's several books of castles:
https://www.amazon.com/Castles-World-Coloring-Dover-History/dp/0486251861
https://www.amazon.com/Castles-World-Coloring-Dover-History/dp/0486251861

I ran across some images from this one today:
https://www.amazon.com/Romantic-Country-Fantasy-Coloring-Book/dp/1250094461

I got this one last week and am pleased with it:
https://www.amazon.com/Medieval-Tapestries-Coloring-Dover-Fashion/dp/0486436861

I don't know if you're a fan, but there's one for Game of Thrones
https://www.amazon.com/gp/product/1101965762/

Here's some free coloring pages
http://www.medievalists.net/2012/09/coloring-pages-about-the-middle-ages/