The tower of Orthanc is said to have been impenetrable:
By the next day, only the tower of Orthanc remained, as it was hewn of one piece of impenetrable rock...
It survives an attack on it by the Ents and has seemingly lasted many thousands of years - it was built in the Second Age.
It is also said that even though it was the Numenorean men who built the tower, it was as though something else was involved:
it was fashioned by the builders of old who smoothed the Ring of Isengard, and yet it seemed a thing not of the craft of Men, but riven from the bones of the earth in the ancient torment of the hills.
What makes the tower so impenetrable?