They differ essentially from all other constructions, because they are adaptations of naturally elevated sites for defensive purposes. The natural site is the defensive position, and the fort itself derives its form and in many cases also its character of construction from the form and nature of the eminence or promontory on which it is built. It would, therefore, be contrary to the nature of the circumstances to expect that they should exhibit any such uniformity of plan or structure as is so conspicuous in the case of the Brochs. Yet it is clear, from the little we do know of them, that there are certain groups possessing certain features of construction in common which differ from other groups possessing other features of construction in common; and it is evident that if a sufficient body of available materials existed on record regarding the different members of these groups their typical characteristics might be readily deduced. But before this can be done with that precision and certainty which are requisite for scientific work, it is necessary (1) that a series of plans and sections to scale of a sufficient number of examples from each of the various groups should be obtained; (2) that a series of observations as to the methods of construction employed in different circumstances and situations should be made; and (3) that a series of examinations of the enclosed areas and surrounding ground should be undertaken, with the view of ascertaining the character of the relics that are associated with the structures. In the meantime it is only possible to indicate some of their general characteristics as exhibited by a few of the better known examples. They naturally divide themselves into two great classes by their construction—(1) those that are earth-works; and (2) those that are constructed of stone.