"Why is this house called a Castle?"
Chillingham Castle, Northumberland which dates from the 12th Century
Creative Commons Attribution Share AlikeChillingham Castle, Northumberland which dates from the 12th Century - Credit: Richard Thomson, Wikimedia Commons

The term castle is generally used to describe fortified* buildings of various types which included a residence.

However, even after the period of English castle building (in its strictest sense) came to an end, the term continued to be used for some country houses, such as Castle Howard, the stately home in Yorkshire on which Brideshead Castle is partially based.

* strengthened against attack