Orthanc was originally built by the Dunedain when they came to Middle Earth and the kingdoms of Arno

Orthanc was originally built by the Dunedain when they came to Middle Earth and the kingdoms of Arnor and Gondor were established.The tower was constructed out of four piers of stone and then hardened by an unknown process. No known weaponry or magic could harm it that existed on Arda. However, the ents that besieged Isengard during the War of the Ring managed to inflict slight damage on the tower, but their efforts ultimately proved to be futile. When Grima threw the Orthanc stone it fell on the stairs; the rail snapped and part of the staircase broke. The only entrance to the tower was reached by a stair of twenty-seven steps, the door could only be opened with the Keys of Orthanc. Above the door there was a balcony, and many tall windows higher up. After the Second Age, the tower was gradually abandoned and came to be occupied by the Dunledings in the Third Age. When the Rohirrim cleared them out, Saruman offered to take the place, which Gondor and Rohan accepted. After the War of the Ring, Aragorn searched Orthanc and found out heirlooms of Isildur, and also the information that Saruman had found his remains and burnt them.