Utumno held out for so long because it was vigorously defended, likely both physically and preternaturally though we have little detail, and was designed to be defended. As to whether it was the strongest fortress in Arda, I might guess that some might nominate Valinor, protected by an entire range of the tallest mountains in existence. And, like any fortress, no impregnable.
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Beleriand, Beleriand,
the borders of the Elven-land.
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