Unnamed Memory, Vol. 1 (manga) — Kuji Furumiya

Once upon a time in a faraway land, a beautiful witch lived atop a tall tower with no intention of descending. Instead, she awaited the brave knight who would overcome many perils to meet her. Should someone triumph over her many traps, she would grant them any one wish. One day, such a courageous prince did arrive, and he asked her to rid him of the curse preventing him from siring an heir. But the curse of another witch cannot be so easily lifted, and the only way to save his bloodline... is to become his bride?!