Thangorodrim was said to have been the piles of slag from Morgoth’s furnaces and rubble from the delving of Angband, but at the same time the mountains were solid enough to form sheer precipices; Maedhros was nailed to a cliff of Thangorodrim, and Húrin imprisoned on a high terrace. The tops of Thangorodrim perpetually smoked, and sometimes spewed forth lava. The three peaks of Thangorodrim functioned as furnaces for Morgoth’s great smithies deep in Angband.
For a time the Eagles lived on Thangorodrim, but at some time during the First Age they moved to the Crissaegrim near Gondolin.
At the base of the south face of the middle peak was the Great Gate 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.