Orthanc

[Celebaglar]

Originally Posted by dancing spawn of ungoliant

From the Unfinished Tales, 3rd age, ch 1...

After king Elessar had been crowned in Gondor, he started to rearrange his kingdom and among the first tasks was the re-building of Orthanc. There he was going to place the palantir of Saruman.
...
Elessar and his men found many treasures (robbed from Edoras) there and the lost white stone, Elendilmir was there also.

The text above is not straight quoted from the book due to the lack of an English copy.

Probably Orthanc become settled by some gondorian guards or something.

I believe that Treebeard stated that Isengard was to grow many things, and become beautiful as it were a garden. I think he meant that it would be reclaimed by the trees, of the fate of Orthanc, I can give only my oppinion and best guess that it would become forgotten or abandoned, like Weathertop or Amon Hen.

[Gil-Galad]

it probaly just got absorbed in Fangorn and stood as reminder for the fury of the ents...