You have it Beachbum and a very concise answer. It was constructed as a mausoleum for the Colebroke family, whose remains were housed in a series of chambers built into the thickness of the walls radiating from a central circular room dressed with decorative columns. When demolished, their remains were interred in a subterranean vault in the churchyard.
With regards to the two castles MartinR, most people regard the castle as the later earlier seventeenth century house and gardens. However, very close by is an unusual hexagonal early Norman keep on an earthen mote built over the remains of an earlier stone building.
I think you have it Beachbum.