Dunharrow was a mysterious place in Rohan. When the Rohirrim were first ceded their lands, they found a number of fortifications created by the Gondorians, but Dunharrow was different - it wasn't created by anyone in Gondor.

It was later used as a fortified refuge by the Rohirrim, hidden in the deep vales of the White Mountains.
Encyclopedia entry originally written by atalante_star