The chief peculiarity of Rutupiae is the presence of a solid mass of masonry underground, a little to the east rather than in the middle of the enclosed space. Many different theories have been put forward to account for its purpose, but it is now generally agreed that it was intended to serve as the foundation for a lofty structure, perhaps of timber, the purpose of which was for signalling between this station and that at Reculver, and possibly also answering to the pharos at Dover. It is not improbable that it also served as a lighthouse for ships entering the estuary of the Stour from the sea. If lights or signals could be seen as far as Dover they might from that point be communicated easily to and fro from the coast of France from the high ground on which the pharos of Dover stands. In order to understand the functions and relative positions of Regulbium and Rutupiae as coast fortresses during the Roman period, it is necessary to reconstruct the ancient geography of the north-eastern part of Kent. The small stream now falling into the sea near Reculver was at the period under consideration a river sufficiently broad and deep to afford a convenient channel for shipping. It was known as the Wantsum. Boats and ships voyaging from the French coast as well as from the British coast near Dover to London, usually took their course through the channel formed by the Stour and the Wantsum, thus avoiding the strong currents and tempestuous seas often raging off the North Foreland. [Illustration: FIG. 8. RECULVER From a print published in 1781] It will be seen, therefore, that a lofty tower or lighthouse at Rutupiae would have been of the greatest value both for the guidance of friendly shipping and as a means of giving warning of the approach of the enemy. The north wall of the castrum at Richborough is a remarkably perfect and interesting specimen of Roman masonry. It is noteworthy, too, as furnishing proof of the great care and thoroughness with which the Romans carried out their building works. At the base of the wall, on the outside, one sees four courses of flint in their natural form, and above them the following succession of materials, in ascending order: three courses of dressed flint; two courses of bonding tile; seven courses of ashlar and two of tile; seven courses of ashlar and two of tile; seven courses of ashlar and two of tile; seven courses of ashlar and two of tile; eight courses of ashlar and two of tile; nine courses of ashlar. The wall is 23 feet 2 inches high, and 10 feet 8 inches thick.