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- All the stone-circle had been thrown down and removed, and the land within was made into a garden filled with orchards and trees, and a stream ran through it; but in the midst of all there was a lake of clear water, and out of it the Tower of Orthanc rose still, tall and impregnable, and its black rock was mirrored in the pool.6.6 -- Book 6 Chapter 6 -- Many Partings (33% in)
There are no more uses of "impregnable" in The Return of the King.
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