The area enclosed is no less than 45 acres, while the whole fort occupies a space of 115 acres. The circumference of this vast work measures one and a half miles, and three enormous valla and fosses stretch this distance; in many places the crest of a vallum above the fosse beneath it amounts to 60 feet. But perhaps our chief admiration is evoked by the complex arrangement, by means of which the two entrances into the fort are protected. A glance at the plans illustrating these will at once show that fortified mounds and bastions of the most complicated forms are placed so as to impede the progress of stormers, and there can be no doubt that every means of protection known at the time were interposed between them and the besieged. [Illustration: FIG. 1. STOCKADE OF STONE AND RUBBLE, WITH PALISADE OF WOOD.] And here perhaps we may mention that the defences of an ancient earthwork can hardly be judged adequately at the present time without imagining the subsidiary structures which once crowned the works. These auxiliary aids cannot with certainty be described, because of the perishable character which generally signalised them, and the very meagre references which occur in the most ancient of our writers. It is generally accepted by authorities upon the subject that some stockade or other defence was invariably added to the summit of a rampart, and that this depended in character upon the nature of the country. In districts where stone was abundant, uncemented walls of large blocks were erected, generally with battering surfaces, the hollow portion between the two faces being filled up with earth or rubble as in Fig. 1. More primitive still would be the single wall with a bank of retaining earth behind it for support (Fig. 2), while more complicated would be one strengthened by a central core of masonry (Fig. 3). Remains of these walls have been found in various places still _in situ_. It is quite possible that a palisade of sharpened stakes or of wattle surmounted these stone walls, thus still further adding to their efficiency. In a "soft" country, where only earth or chalk is available, timber would naturally take the place of stone. The Gallic defences of this nature, which gave so much trouble to Caesar's legions, appear to have been made of tree-trunks lying side by side upon the ground with the second course of trunks superposed at right angles, the whole of the interstices being filled with stones and earth tightly rammed (Fig. 4). It will readily be perceived that a rampart constructed of alternate courses similar to this, and approximately 10 feet in thickness and of considerable height, would be quite impervious to the missile weapons of the period, and indestructible by fire, even if the assailants succeeded in filling up the deep vallum below the base of the wall with combustible materials. Whether this method of the utilisation of timber for barricades was ever introduced into the British Isles for strengthening valla we have