Database
Unfinished Tales Of Númenor and Middle-earth
In the wall was a great archway above the road, but it seemed that masons had blocked it with a single mighty stone...
'Here stands the Second Gate, the Gate of Stone,' said Elemmakil; and going up he thrust lightly upon it. It turned upon an unseen pivot, until its edge was towards them, and the way was open upon either side...
—p.47
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Now Amon Ethir was a mound as great as a hill that long ago Felagund had caused to be raised with great labor in the plain before his Doors, a league east of Narog. It was tree-grown, save on the summit, where a wide view might be had on all ways, of the roads that led to the great bridge Nargothrond, and of the lands round about.
—p.116
This is awesome. So awesome. Also, it shows that the Noldor of Gondolin were very accomplished stoneworkers and engineers.
This sounds similar to Silbury Hill, just much larger.