Greenstone is the seat of House Estermont on the island of Estermont in the stormlands.
King Robert I Baratheon along with Queen Cersei Lannister and some of the court from King's Landing, including Ser Jaime Lannister of the Kingsguard, spent a fortnight at Greenstone. Robert cheated on Cersei with a cousin of his and Cersei believes that it was there that she and Jaime conceived their eldest child Prince Joffrey. Jaime gave the castle the nickname "Greenshit".
A Dance with Dragons
A force of the Golden Company under the command of Marq Mandrake takes over Greenstone.