Traditions and Hearthside Stories of West Cornwall, Vol. 2, by William Bottrell, , at sacred-texts.com
It is not known what powerful magician raised this giant's hold, though it was believed that its security depended on a magic stone called "the key of the Castle," respecting which Merlin had something to say, as well as about many other remarkable stones in the neighbourhood. Castle Treen, however, must have stood where it is long before Arthur and his magician visited West Cornwall.
The key was an egg-shaped stone, between two and three feet long, which was contained in the cavity of a rock with a hole facing the sea, through which it might be turned round; and the opening appeared large enough for it to be passed through. Many attempted to get it out, but they always found it to hitch somewhere; and lucky (according to old folks’ faith) that it did, because the sage Merlin prophecied that when the key of the Castle was taken out of the hole, Men Amber (the holy rock) would be overthrown, the Castle sink beneath the ocean, and other calamities occur.
The key was situated near the bottom of a deep chasm called The Gap, which is passed on approaching the Logan Rock by the usual path. It required a sure-footed climber, of strong nerve, to reach it, and this could only be done from land, at low water, or nearly so.
Surging waves occasionally changed the position of this magic stone, and from the direction of its smaller end, as it lay in a trough of water, prognostics were drawn with regard to the seasons, &c.
Few persons had sufficient hardihood to descend the precipitous cliff and risk being caught in The Gap by a flowing tide; and the key of the Castle remained a mysterious and venerated object till Goldsmith's mischievous tars, or the dockyard men who were employed in erecting machinery to replace Men Amber (as the stone they overthrew was formerly called) heard of it and the traditions connected therewith. Then, one day, some of these wretches, on farther mischief bent, entered The Gap in a boat, and, being provided with crow bars, they broke away the edges of the rock that enclosed the key, ripped it out, and tumbled it down among the sea-washed pebbles! Some calamity has surely befallen these wretches ere this, or Bad Luck is a mere name, and powerless as an avenging deity.
Part of Merlin's prophecy was fulfilled, however, yet not in the order predicted.
The venerated nodule was what is called, among miners, at "bull's eye," or "pig's egg," of large size. It appeared to be a closer-grained and harder stone than what surrounded it.