You have it shoot999. It is indeed the main eastern gateway to Saltwood Castle. Regarded as the location of the plotting by the four knights whom later killed Thomas Beckett at Canterbury Cathedral on 28th December 1170.
Formerly the home of both Bill Deedes MP and later Kenneth Clark MP and after, his son Alan Clark MP who was buried in the grounds. Formerly the grounds, and parts of the castle, including the great hall, were open to the public, but sadly no longer. It is still possible to view the outer walls from public footpaths that run alongside though.
Over to you shoot999