From A Wiki of Ice and Fire
Oldstones is the name given to the remnants of an ancient castle in the Riverlands that once belonged to House Mudd. Its original name has long been lost. The local smallfolk gave it its new name of Oldstones. 
The ruined stronghold sits on a hill above the Blue Fork of the Trident. Nothing but its foundations remain and a sepulcher of the ancient River King Tristifer IV Mudd.  The curtain wall of Oldstones once encircled the brow of the hill upon which it sits, like the crown on a king’s head. However only the foundation now remain and a few waist-high piles of crumbling stone spotted with lichen. 
Its foundations remain amongst the weeds to show where the walls and keeps had once stood, but the local smallfolk had long ago made off with most of the stones to raise their barns and septs and holdfasts.
Where the gatehouse once stood the ruins are more extensive and a rider must dismount to lead their horse through them. Once inside the vanished walls the weeds are chest high.
Beneath the castle ruins, the lower slopes of the hill are thickly forested. There are gorse, bracken, thistle, sedge and blackberry bushes between pines and grey-green sentinels. Elsewhere skeletal elm, ash and scrub oaks choke the ground like weeds.
The road up to Oldstones goes twice around the hill before reaching the summit.  It is overgrown, stony and is slow going even in the best of times.
At the center of what once would have been the castle’s yard, a great carved sepulcher still rests, half-hidden in the waist high brown grass amongst a stand of ash trees. The sepulcher entombs Tristifer, the Fourth in His Name, King of the Rivers and the Hills. 
The lid of the sepulcher was carved into a likeness of the man whose bones lie beneath, but the rain and the wind has worn down the carving. Catelyn and Robb can see that the king had worn a beard, but otherwise his face is smooth and featureless with only the vague suggestions of a mouth, a nose, eyes and the crown about his temples.
The king’s hands are folded over the shaft of a stone warhammer that lies upon his chest. Once the warhammer would have been carved with the runes that told its name and history, but the runes have been worn away.  The stone itself is cracked and crumbling at the corners and discolored here and there by spreading white splotches of lichen, while wild roses creep up over the king’s feet almost to his chest.
A Storm of Swords
On the way to the Twins Robb Stark encamps upon the hill overlooking the Blue Fork, within the ruined stronghold castle of the ancient river kings. Catelyn Tully finds him standing somberly by the sepulcher.
Catelyn remembers that she had camped at Oldstones once with her father, on their way to Seagard.  She recalls that Petyr was with them too. When Robb asks her if the castle has a name she tells him that all the smallfolk called it Oldstones when she was a girl. Robb then remembers that there was a song about Jenny of Oldstones. Catelyn thinks to herself that she had played at being Jenny that day and had even wound flowers in her hair. And Petyr had pretended to be her Prince of Dragonflies. She thinks they could not have been more than twelve, Petyr was just a boy. Robb discusses whom he will name as his successor with her, as he and his wife not yet have children. Against his mother’s wishes he tells her that he plans to legitimise his brother Jon Snow and name him his heir. 
Much later, after the Red Wedding has taken place, Merrett Frey travels to the ruins of Oldstones with the ransom for his half-brother's grandson Petyr Pimple. When he enters he hears faint music coming drifting through the trees from somewhere deep within the castle. As he reaches the site of King Tristifer’s sepulchre, finds Tom of Sevenstreams sitting cross-legged atop it, fingering the strings of his woodharp. he quickly encounters the rest of the outlaws who had captured Petyr. They surround him, take the gold he carried for the ransom and hang him. 
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