St Mary de Castro - Dover Castle. - Medieval Churches on Waymarking.com

Records exist of a church being built here in 630 by King Eadbald . The present church of St Mary de Castro is a cruciform, its tower being the same width as its nave. It has the earliest known surviving door arch of any standing church in England. The church sits on the cliff tops within Dover Castle and is thought to have been built in the late Saxon period (1000 AD).  However, it is know that it was preceded by a Roman building, probably having some connection with the manning of the Roman Lighthouse (Pharos) alongside the present church building.  Roman tiles and building material from that building seem to have been incorporated into the walls and arches by the Saxons.

Work on the church has been seen many alterations and additions. As far back as 1226, Henry III instructed that the church be repaired and added 3 new altars.  The ancient Roman Lighthouse, built soon after the 1066 Norman invasion, was even made into the church's bell tower in 1262 when 3 bells were hung in the strengthened pharos. They in turn were replaced in 1345 by 2 much heavier bells. From the Saxon period till present day, St Mary de Castro has been and remains fore most a garrison church, serving the local Army barracks, albeit somewhat depleted these days, but also the local community. Infact,  if you are attending a service being held at the church, you are not required to purchase a ticket into the castle itself, which is run by English Heritage.