@BalrogRingDestroyer I don't know that it was ever mentioned that the builders of
Orthanc willingly gave it to
Saruman. It was built my the
men of
Numenor during the height of their power during the
Second Age and later abandoned or overrun, as so many of their strongholds later were. The
Istari didn't arrive in M-E until the beginning of
the Third Age, and by the time that Saruman found and took Orthanc for his own it would have been empty for some time.
At least that is how I understood it.
The Appendix says Saruman was given the keys to Orthanc as a friend in T.A. 2759.
Rohan was behind that, were they not?
Uh, no, to both of you.
After
Gondor gave Calenadhorn to Rohan, Orthanc remained a part of Gondor with Gondorian garrison there - likely due to
Palantir that was in it. But Gondor, being busy with threats from
the East, soon forgot the Orthanc even existed. Garrison continued existing as a hereditary community, from father to son, but Gondorian troops there increasingly intermarried with
Dunlendings. So eventually, they became closer to Dunlendings than to
Rohirrim and let them inside
the ring as friends - which Dunlendings used to take over the
Isengard. Long story short, they caused a lot of trouble and were eventually chased out - and with Isengard now empty, Saruman came, seeking Palantir, and
Steward of Gondor at the time Whatshisname gave him the keys to Orthanc and Isengard.
In short: Orthanc was built by Gondor and since Gondor gave Orthanc to Saruman, you
could say that the "builders of Orthanc willingly gave it to Saruman". But Orthanc was
technically never abandoned, it was overrun, and yes, it was empty by the time Saruman was given keys to Orthanc. But it wasn't Rohan who was behind it, it was done on initiative of Gondor, though it is difficult to imagine Rohirrim would have objected to having (as it was then believed) a powerful ally in control of strategically incredibly important
fortress.