A more complex variety consists of two, three, or more walls drawn concentrically round the upper part of a conical hill, at short distances apart, as in the case of the example known as the White Caterthun in the parish of Menmuir, Forfarshire. The area enclosed is a long oval about 450 by 200 feet. The enclosing wall has been of enormous size. Its remains have spread themselves over a width in some places of nearly 100 feet, and they now form a somewhat rounded embankment of from 4 to 6 feet high, encompassing the summit of the hill. About 150 feet lower down on the slope of the hill is another wall, equally ruined, and below it are the remains of a third. Beyond this there is an enclosure of an oblong form and of less massive construction, abutting against one side of the outer wall of the fort.[99] A fort of smaller size, but presenting somewhat similar features of construction, crowns the spur of Ben Ledi which overlooks the ford at Coilantogle, a little below the outlet of Loch Vennachar. The hill is precipitous on one side, and the walls do not encircle it completely, but the external faces of three encircling walls are in some places visible for a considerable distance round the less precipitous part of the hill. Abutting on the outer wall, on the side which is most accessible, is an oval enclosure less massively constructed, as at Caterthun.
Such constructions are frequently found in similar juxtaposition to the walls of these forts, and rightly or wrongly they have been regarded as cattle-folds.
There is another variety of these Hill-Forts which has attracted more attention on account of the singularity of the phenomena which they present. These are the Vitrified Forts, so called because in their walls there is always more or less of the scorified or vitrified appearance which is the result of the action of fire upon masses of loose stones. Although there has been perhaps more written about these singular structures than about any other class of antiquities, there is really little known of their special phenomena, and less of their real character.[100] In point of fact the real knowledge relating to the form, measurements, and composition of the structures and the observation of the phenomena they present has been entirely overlooked in fruitless discussions as to the modes in which the vitrifaction of the walls has been produced, and the reasons which may be conjecturally assigned for it. The result is that to this day, so far as I am aware, there is not a single scale-plan with sections, of a single one of them. When such plans and sectional drawings are available in sufficient numbers, we shall be able to say that the materials exist for the commencement of a systematic investigation of the nature and typical relations of the structures.