Together they went to a breach in the ponderous walls and peered down into the ancient river-bed, now but a rippling burn.

“Very well,” replied Murie, “let us descend and explore.”

So they retraced their steps until, when about half-way to the house, they left the path and went down to the bottom of the beautiful glen until they were immediately beneath the old castle.

The spot was remote and seldom visited.  Few ever came there, for it was approached by no path on that side of the burn, so that the keepers always passed along the opposite bank.  They had no necessity to penetrate there.  Besides, it was too near the house.

Through the bracken and undergrowth, passing by big trees that in the ages had sprung up from seedlings dropped by the birds or sown by the winds, they slowly ascended to the frowning walls far above—­the walls that had withstood so many sieges and the ravages of so many centuries.

Half a dozen times the girl’s skirt became entangled in the briars, and once she tore her cape upon some thorns.  But, enjoying the adventure, she went on, Walter going first and clearing a way for her as best he could.

“Nobody has ever been up here before, I’m quite certain,” Gabrielle cried, halting, breathless, for a moment.  “Old Stewart, who says he knows every inch of the estate, has never climbed here, I’m sure.”

“I don’t expect he has,” declared her lover.

At last they found themselves beneath the foundations of one of the flanking-towers of the castle walls, whereupon he suggested that if they followed the wall right along and examined it closely they might discover some entrance.

“I somehow fear there will not be any door on the exposed side,” he added.

The base of the walls was all along hidden by thick undergrowth, therefore the examination proved extremely difficult.  Nevertheless, keenly interested in their exploration, the pair kept on struggling and climbing until the perspiration rolled off both their faces.

Suddenly, Walter uttered a cry of surprise.  “Why, look here!  This seems like a track.  People have been up here after all!”

And his companion saw that from the burn below, up through the bushes, ran a narrow winding path, which showed little sign of frequent use.

Walter went on before her, quickly following the path until it turned at right-angles and ended before a low door of rough wood which filled a small breach in the wall—­a breach made, in all probability, at the last siege in the early seventeenth century.

“This must lead somewhere!” cried Walter excitedly; and, lifting the roughly constructed wooden latch, he pushed the door open, disclosing a cavernous darkness.

A dank, earthy smell greeted their nostrils.  It was certainly an uncanny place.

“By Jove!” cried Walter, “I wonder where this leads to?” And, taking out his vestas, he struck one, and, holding it before him, went forward, passing through the breach in the broken wall into a stone passage which led to the left for a few yards and gave entrance to exactly what Gabrielle had expected—­a small, windowless stone chamber probably used in olden days as a dungeon.