Castle de Sade is the name of a large manor set where the Yarrow meets Dokrayth, between Caerleon and Aeswick. It is the ancestral home of the Sartres, nobility that has pledged allegiance to the royals of Soristine.
The Sartres pride themselves on their connections to other families of noble birth for their lineage cannot be traced back as far. They can be traced back 'far enough' and have enough of a pedigree to stand with their fellow nobles. The last patriarch was Etienne Sartre, but he and his family have abandoned the estate in lieu of the fall of Dokrayth. It is unknown if any survived.
The estate is well-known for the lavish gardens and integrated architecture it possesses. There is a myth that the gardens share soil with the Yarrow, and that is why the gardens are so extravagant.