This area has been revamped into a reputation area for The Eglain, and split into 4 areas, 3 that are now separate instances, that no longer require bind on acquire drop keys to enter.
"Garth Agarwen . . . The ruins of an ancient fortress of Rhudaur, now overrun by corrupted nature and a vile necromantic power out of Angmar. The most terrible power still remains to be confronted."
Head to the Red Pass, the area where you appeared after completing Book II, Chapter 8.
At the Red Pass there are quests given for Agamaur (the outside area) and Garth Agarwen (the instance): 2 of the Agamaur quests need to be completed for the last Garth Agarwen quest.
Head east from the Barad Dhorn, the Free Peoples outpost, and go through the gate. Head over the ruins (similar to those encountered near the end of Book 2, Chapter 8), and a little north-west is a stairway leading to further areas. Proceed through there, then head east past the Creoth encampments, staying near the north wall if possible. Shortly thereafter, an arrow should appear on your mini-map, pointing you towards the instance entrance.