Fornost

In the North after the war and the slaughter of the Gladden Fields the Men of Westernesse were diminished, and their city of Annúminas beside Lake Evendim fell into ruin; and the heirs of Valandil removed and dwelt at Fornost on the high North Downs, and that now too is desolate. Men call it Deadmen’s Dike, and they fear to tread there. For the folk of Arnor dwindled, and their foes devoured them, and their lordship passed, leaving only green mounds in the grassy hills.
– The Fellowship of the Ring (Book 2), Chapter 2

Likely built by the Northern Dúnedain in the late Second Age, Fornost (or Fornost Erain) was the largest human settlement in the north-west of Middle-earth for much of the Third Age. After the abandonment of Annúminas in T.A. 861, it became the capital of the kingdom of Arnor and later of Arthedain, one of its successor kingdoms.

Only very little remains of the once mighty city now, as much of it was destroyed in the Witch King’s capture of the city. After his defeat, the remaining Dúnedain did not rebuild their former capital, leaving its remains to decay. Its ruins were said to be haunted with no one but the Dúnedain visiting what was now commonly known as Deadman’s Dike.