There are a surprising number of Castles, historical Houses, and open Gardens in Cornwall!
"In 1644, Lanhydrock House was the headquarters of the Parliamentarian cause in Cornwall, but fell to the Royalists later that year, until once again returning to the Parliamentarians when the Royalists lost the war in Cornwall and the west in 1646. Pendennis Castle was the last Royalist stronghold in the county to surrender, and after a five month seige, the defenders were allowed to march out with full military honours, weapons, and banners flying."
Everyone has heard of the Eden Project, but this is a 21st century development (see SE Cornwall - Inland for this one!), and not really a historical house or garden. But there are many houses and gardens that came to prominence over the past 200 years or more, and some houses established well before then, and then a good number of castles that were built over 500 years ago and some very much earlier. And then there are those that were lost, and have been found again - such as a certain one at Heligan!