It is in White Harbor.
It has crumbling black walls, and looms over the houses that are attached to them.
The Wolf's Den was built by King Jon Stark, and was frequently given to brothers, cousins, uncles and younger sons of House Stark to hold. Some of these passed it onto their sons, and cadet branches of the Starks started to form. The longest living of these houses were the Greystarks, who held it until they rose in rebellion against Winterfell.
After the extinction of the Greystarks, the castle swapped hands many times. The Flints held it for a century, and House Locke held it for two. Houses Ashwood, Holt, Long and Slate would all be tasked with holding the castle. The castle was once taken by raiders from the Three Sisters, and was besieged by King Osgood Arryn, and burned by his son, and burned by his son, Oswin Arryn, during a war between the North and the Vale of Arryn.
Slavers from the Stepstones captured the Wolf's Den under the rule of King Edrick Stark, as he had become too feeble to defend hois kingdom. Winter descended, and the White Knife froze around the slaver's ships. Whilst they were huddled around their fire, the new King of Winter, Brandon Stark, known as Ice Eyes, attacked them, and then, when the slavers were defeated, gave them over to the captured slaves, who hung their entrails in the trees as an offering to the Old Gods.
A thousand years before the War of Conquest.House Manderly was given the Wolf's Den by House Stark after they fled the Reach. They swore oaths before both the Old Gods and the New Gods to serve as loyal vassals, in return for land and protection. The Manderlys eventually built the New Castle, and converted the Wolf's Den into a prison. Lord Wyman Manderly gave the Wolf's Den to Ser Bartimus in return for saving his life during the Battle of the Trident.