“a large building or group of buildings fortified with thick walls, battlements, and often a moat; castles were the strongholds of noblemen in the Middle Ages
2. any massive dwelling somewhat like this”
Personally, and as a Canadian who mostly learned about castles in French, I probably wouldn’t use castle (or chateau) even informally to describe Wyntham Abbey, since it’s not as big (especially as tall) as how I imagine castles to be. My images of ‘manor’ and ‘mansion’ are also smaller and more compact than Wytham Abbey, too, though. ‘Estate’ seems about right.
‘Castle’ has definitions that don’t require military fortification, and could often be used less formally that way (maybe more so in North America?):
“a massive or imposing house” https://www.merriam-webster.com/dictionary/castle
“a large magnificent house, esp when the present or former home of a nobleman or prince” https://www.collinsdictionary.com/dictionary/english/castle https://www.dictionary.com/browse/castle
“a large building or group of buildings fortified with thick walls, battlements, and often a moat; castles were the strongholds of noblemen in the Middle Ages 2. any massive dwelling somewhat like this”
“a large and stately residence, esp. one, with high walls and towers, that imitates the form of a medieval castle” https://www.collinsdictionary.com/dictionary/english/castle
You need to scroll a bit for some of these alternate definitions, though.
‘Mansion’ and ‘estate house’ are listed as synonyms here, too: https://www.thesaurus.com/browse/castle
Personally, and as a Canadian who mostly learned about castles in French, I probably wouldn’t use castle (or chateau) even informally to describe Wyntham Abbey, since it’s not as big (especially as tall) as how I imagine castles to be. My images of ‘manor’ and ‘mansion’ are also smaller and more compact than Wytham Abbey, too, though. ‘Estate’ seems about right.