In the First Age, the entrance to Nargothrond consisted of a pair of great doors, raised by Finrod Felagund. They were set in the face of a cliff above the river Narog, and could be accessed only by a narrow pathway, keeping Nargothrond secure from enemy attack.
After Finrod's death, Túrin came to dwell in Nargothrond, and persuaded Finrod's successor Orodreth to build a great bridge before the Doors of Felagund, so that his armies could more easily pass over the Narog. This proved to be a grave mistake, as it gave Morgoth's forces a means to approach the Doors. Following the Battle of Tumhalad, the dragon Glaurung came against the Doors and destroyed them, allowing the Orcs to sack Nargothrond.