Beauty and renown it was the best
and it was built to withstand the fortold test
for peace and splendor it held for many an age
til, in one great swoop, it fell to pillage.
Gondolin
Gondolin the great Hidden City of Turgon was concealed from friend and foe alike during the First Age by the Encircling Mountains, and guarded from trespassers by the Eagles of Thorondor.
Its inhabitants really felt that it was impregnable, but it fell to the onslaught of Gothmog and Co. What of the fortold test? I can't think of any tests foretold for a place in all of Tolkien's work. Ulmo did of course foretell that Gondolin would fall eventually, but that is not a test. I don't think it can be Gondolin, or Nargothrond. Both cities were founded in the First Age ("of the Sun", that is, as opposed to an "Age of Valinor"), and fell in the First Age; in fact, I don't think either one lasted as much as a thousand years. (Don't-blink-you'll-miss-it, by Elven standards.) So that nixes the "many an age" in line 3.
Tolkien Gateway: Gondolin