Gate of Isengard
From Tolkien Gateway
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The Gate of Isengard was a strong gateway guarding the single arched tunnel in the wide rock-wall that surrounded Isengard and Orthanc. It lay in the southern part of Isengard's defences, approached by an ancient highway. At least in Saruman's time as master of Isengard, the gate was heavily guarded, though it was eventually thrown down by the invading Ents.
According to some background material, Saruman had given his Gate special properties. Whether by some mechanism or spell, he was able to project his famous voice through the Gate itself, and so communicate with those outside his defences without putting himself at risk.