Windsor. Next one might be a challenge, unless that pleasure dome at Xanadu was a castle.
One man's stately pleasure dome is another man's castle.
It had (I'm told) walls and towers girding a considerable acreage of land with very real development potential in a rising market. That sounds like a castle to me.
It also boasted an unusual ornamental garden and a rare line in Abyssinian maids with a musical gift.
Only the harshest judge would question the inclusion of Xanadu.
I say it's allowable and on that basis I'm using it:
Xanadu Stately Pleasure Dome.