From the humanistic point of view every human achievement is unforgettable and immortal in its essence, even if it is replaced by a "better" one.
Perhaps the main, as well as the least obvious, achievement of the Middle Ages, was the creation of the experimental spirit, or more exactly its slow incubation. This was primarily due to Muslims down to the end of the twelfth century, then to Christians.
Scientific achievements seem evanescent, because the very progress of science causes their supersedure; yet some of them are of so fundamental a nature that they are immortal in a deeper way.