You can go the DIY route and build modular pseudo-medieval castles using PDF castle designs.
Album of my paper creations. I printed mine on regular paper, using my color laser printer; I glued the paper with a Glue Stick to 3mm-thick chipboard; the chipboard was assembled using Hot Glue (PVA Glue is too slow, too tedious...) and wooden square and round dowels to reinforce and prevent warping. Some of my wall sections and Towers are 15+ years old, and they look the same as when they were built.
The really
large gatehouse (on the left side, laying down) was made by adapting FDG castle printouts to a large, almost octagon-shaped, oatmeal box. I had to piece sheets together, but it turned out to be good enough to game with.

I further adapted the FDG square tower sheets by wrapping them around cardboard tubes, to make
round towers. The merlons were made by gluing the merlon printouts to both sides of toilet paper tubes, cut to shape, and then glued to the tops of the tower tubes. The steel doors are from a Dungeon 2D PDF, which featured the door designs to be made as stand-up's. I printed these, gluing them to the tower bases, to create a seamless, heavy iron door. When based, they make a marvelous
Wizard's Tower for adventurers to explore.
I've used them many times in my RPG and my fantasy mass battles games.
Another album: the first photos are from games, but below that are the same photos as in the previous album. I even made
chipboard Siege Towers (there are some images of the first one, made of 110# cardstock, and how warped it became before I pitched it...).
The most amazing thing about the chipboard constructs?
No painting! I do use a black brush-tipped acrylic paint pen to edge the chipboard and the paper, to erase the white edges. This is quick, efficient, and easy.
Many people apply the PDF printouts to foamcore sheets, cutting/assembling them that way. This is a very good way to make stronger, heftier models, as well. It requires slight alterations from my methodology, but it works quite well also.
Any of the amazing 28mm fantasy building PDF's can be adapted to my methods. I build mine from Fat Dragon Games, TSR (were free on the Internet, decades ago), and even Mage Knight stuff (were free on the Internet, decades ago, as well). You can find such PDF buildings on
DriveThruRPG.com (my search yielded
463 products, spread across 16 pages, to review!

). My personal favorites are
David Graffam models, and
Fat Dragon Games sets (my castle is from FDG).
Not what you are looking for, but it does give you a truckload of options to at least consider... Cheers!