Another fact is apparent when dealing with this subject: the earthwork is much more durable than any other form of castrametation, in fact it is almost indestructible so far as meteoric agencies are concerned, whereas the parapet suffers not only from disintegration by the weathering influences of rain, frost, wind, and heat, but also from the tendency to lose its original shape through having no natural or artificial coherence between the separate parts. Thus undoubted examples of prehistoric ramparts are comparatively rare when compared with the wealth of existent earthworks. It must be borne in mind that the study of the earthwork is the alphabet to that of castellation, and that the evolution of the latter cannot be efficiently comprehended without an intelligent appreciation of the former. So far as classification of earthworks has been made to the present time, the following table represents the general mode of procedure, and under one or other of its separate headings the whole of the earthworks, so far as our knowledge extends at the time of writing, may be allocated. CLASSIFICATION OF EARTHWORKS 1. _Natural Fortresses strengthened._ This refers to fortresses partly inaccessible by reason of precipices, cliffs, or water, additionally defended by artificial banks or walls. 2. _Fortified Hill-Tops strengthened._ This includes fortresses situated on hill-tops, with artificial defences adapted to the natural configuration of the ground, or to those which are less dependent on the natural slopes. 3. _Simple Artificial Enclosures_, including rectangular or other forms, and all the fortifications and towns of the Romano-British period. 4. _The Mount and Fosse._ 5. _The Mount and Bailey_, consisting of natural or artificial mounds with one or more courts attached. 6. _Homestead Moats._ 7. _Homestead Moats developed_, referring to enclosures similar to No. 6 but augmented by supplementary defences. 8. _Protected Village Sites._ _Class I.--Natural Fortresses strengthened._ This division may very readily be subdivided into three parts dealing with natural fortresses according to the topographical characteristics as follows: (_a_) Promontory forts, or cliff castles both upon the coasts and inland. (_b_) Those depending upon rivers, woods, marshes, etc. for efficiency. (_c_) Plateau forts.