Thangorodrim was a group of three volcanic mountains in the Iron Mountains in the north of Middle-earth during the First Age. The highest peaks of Middle-earth, they were raised by Morgoth, who delved his fortress of Angband beneath them, and far back into the Iron Mountains. At the base of the south face of the middle peak was the Great Gates of Angband, a deep canyon leading into the mountain, lined with towers and forts. There were also a number of secret gates scattered around the sides of the mountain group, from which Morgoth's hosts could issue forth and surprise their foes.
