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According to the text in "The Fellowship of the Ring," Isengard is characterized as a circular stone wall encircling a valley, in which stands the imposing Orthanc tower. Contrary to the popular belief that Saruman constructed Orthanc, it was originally erected by the Dúnedain. This formidable tower reaches 500 feet—approximately the height of a 40-story building—and is said to be made from a mysterious indestructible black stone, immune to fire and steel.
Tolkien describes that even powerful beings like Saruman found the tower's secrets elusive, requiring him to forge alliances to gain access. The tower’s entrance remained untouched, even as invaders attempted to breach it over centuries; many simply chose to dwell within the walls instead.