In 55 AC, the elderly Lord Estermont had been wed to a lady of House Tarth. His wife's widowed mother, Larissa Velaryon, chose to stay with her daughter on Greenstone after the wedding. Princess Rhaena Targaryen visited Larissa on Greenstone, who was the cousin and the first of her favorites.
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 bedded a widowed Estermont 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 Feast for Crows
After the failed coup to crown Princess Myrcella Baratheon, Symon Santagar, the Knight of Spottswood, betroths his daughter and heir, Sylva Santagar, to Eldon Estermont, Lord of Greenstone. He quickly shipped her to Greenstone.
A Dance with Dragons