[TOMT] [MANGA ]Isekai manga is dedicated to the designer.

Solved - World Customize Creator.

I remember that in this manga, the main character is a black-haired man who can use some kind of interface to build quickly. In one of the chapters, he builds a fortress in one day, which greatly surprises the spies of a neighboring country.

Besides, black hair is very rare in the isekai world, and when the princess of the kingdom notices him and his abilities, she forces him to work for her.

Also, at the very beginning, he appears naked in isekai, and some old man sheltered him.

And its not "The Greatest Estate Designer"