Lol. It's not just Korean manhwas. Webtoons in general are usually made in the same software (ClipStudio I believe) and the software subscription comes with a gallery of assets for the artists to use, so they don't have to draw everything and can focus on the characters and more important details. This castle I'm assuming is their asset for Medieval Castle or something similar, and so the authors use it to avoid having to draw the huge building from scratch and be able to deliver chapters faster. It's not only the castle. Assets are repeated across several webtoons all the time. Pay attention next time you are reading any historical webtoon and there's a tea scene, you are almost 100% sure to see a rose pattern teacup in there. It's also super overused.

GUYSSSSSS does anyone know why basically almost every korean shoujo manhwa has the same castle??? Like first I thought it was a coincidence but I've seen it so much that is is starting to bothering me now haha