A spur was constructed on the northern side of the castle in the C13th after the 1216 siege and reflects successive attempts by military engineers to reduce the vulnerability of the northern defences from attack from the higher ground to the north. The bridge towards the spur and its associated tower (St John’s Tower) formed part of the elaborate post-siege strengthening of the northern defences, providing a key link in the communications between the castle and spur. This circular tower was under construction in the middle of the moat in the 1220s and the medieval tower was refurbished in the 1750s.

The precise plan of the medieval Spur remains unknown, but it appears to have been roughly oval in shape. The Spur was approached from the castle by means of a tunnel and bridge that led directly to a chalk tunnel in the rear of the Spur. This tunnel divided into three passages that apparently led to three towers. None of these towers have left any visible trace. Today the Tower and spur is approached by a tunnel descending from a point inside the Norfolk Towers, though this is only one of a series of tunnels re-used after the siege or constructed as part of the renewed defences. The garrison gained access to it through the tunnel cut below the blocked North Gateway that led to a short covered bridge to the south side of the tower. On the northern side of this tower was a drawbridge linking to the tunnel in the rear of the spur. Two doors in the base of the tower allowed defenders to sally into the moat, while the top of the tower allowed archers to overlook the spur. There are considerable remains of the Medieval tunnels inside the later Spur caponiers, and in particular the three-branched section on the north side of the ditch. Other parts of the medieval tunnels are inaccessible. (summarised from sources) (1-2)

<1> English Heritage, 2014, Dover Castle Conservation Management Plan Volume 2 Gazetteer (Unpublished document). SKE52105.

<2> Johnathan Coad, 1995, English Heritage Book of Dover Castle and the Defences of Dover (Monograph). SKE52106.