I picked it up and opened it.

I was all in a tumult, for my heart told me whence it came.
Inside was written in English, in a large, sprawling hand, such as might be from an English child of seven or eight: "Meet me at the Flagstaff on the Rock!" I knew the place, of course.

On the farthermost point of the rock on which the Castle stands is set a high flagstaff, whereon in old time the banner of the Vissarion family flew.

At some far-off time, when the Castle had been liable to attack, this point had been strongly fortified.
Indeed, in the days when the bow was a martial weapon it must have been quite impregnable.
A covered gallery, with loopholes for arrows, had been cut in the solid rock, running right round the point, quite surrounding the flagstaff and the great boss of rock on whose centre it was reared.

A narrow drawbridge of immense strength had connected--in peaceful times, and still remained--the outer point of rock with an entrance formed in the outer wall, and guarded with flanking towers and a portcullis.