Himring

We don't have the exact date that Maedhros settled on Himring, but we do know that he built his fortress shortly after the council in Mithrim that made Fingolfin the High King of the Noldor. The date of that council was I 7, so Himring seems to have been fortified within only a few years of the Return of the Noldor.

After the War of Wrath, most of Beleriand was lost under the Sea. Himring was one of very few exceptions, surviving as the small island of Himling off the coasts of Forlindon. It seems implausible that Maedhros' fortifications would have survived the inrush of the Great Sea.

In chapter 14 of The Silmarillion, Tolkien calls this hill 'Himring, the Ever-cold', which looks like at least a partial translation. This presumably comes from the word him, meaning 'steadfast' or 'abiding', alongside ring, 'cold'.

In his linguistic appendix to The Silmarillion, however, Christopher Tolkien tentatively interprets the him element as meaning 'cool'. While 'cool-cold' seems a rather redundant placename, it could in principle derive from Himlad ('cool plain'), the land to the immediate southwest (making the name Himring a shortening of 'cold hill on the cool plain').