Springvale is a massive, partially-ruined castle in the North, considered to be the third largest in all of Westeros, behind Harrenhal in the Riverlands and Karthmere in the Borderlands. It has been described as the "Harrenhal of the North".
It is located in the direct center of the Valley of the Sun, the southernmost point of the North, and also the location of the Battle of Springvale, the climactic final battle of the Oradun Rebellion, which resulted in the castle being destroyed in the first place. The Battle of Springvale has often been described as the bloodiest battle in the history of the North.
Before its destruction, Springvale was the summer palace of House Stark. It was ruled by the firstborn son and heir to whoever was King in the North at the time. Its ruler was styled as Prince of Springvale.
After it's destruction during the Battle of Springvale, it was left abandoned in its ruined state due to the belief that it had become severely haunted by the ghosts of those who died during the battle.
Years later, during the War of the Five Kings, after Robb Stark had been declared King in the North, he intended to rebuild Springvale and have it under control of House Stark once again. He initially named Ashter Stark as his heir, thus making him Prince of Springvale, but Ashter later declined, promoting Robb to name his other brother, Rodrik Stark, as Prince of Springvale instead.
Following the Red Wedding, the militant group known as Grayburn's Legion was officially formed at Springvale. The group used Springvale as their official hideout for the early stages of its existence, with their leader, Carmine Grayburn, declaring himself unofficially as Lord of Springvale. After Grayburn's Legion captured the Dreadfort, it became the group's new hideout, but Springvale remained under the Legion's control, and is currently still under their control.
With Jon Snow being declared as the new King in the North following his victory during the Battle of the Bastards, it is unknown if he plans to rebuild Springvale and bring back the title of Prince of Springvale like Robb did. If he does, the title would likely go to Sansa Stark as Princess of Springvale.