[Turin]

The Sil is a bit vague on this sort of thing but there are a couple of hints:

Maedhros is said to have done "deeds of surpassing valour" during the Dagor Bragollach. He was an Amanian elf, as well as being the son of the greatest elf of all time, so he must have been very powerful in his own right. The Sil doesn't go into too much detail because it is not really within the main purpose of the story; which is to describe the breaking of the Seige of Angband, and how Morgoth's armies began to overrun northern Beleriand. Although, evidently it must have been a great deed since Tolkien makes a special mention of Maedhros' valour. Himring was quite isolated and looked down towards large areas of plain, making it quite vulnerable.

Without being able to refer to my book, I think that Maglor held the Gap, and when he was being overrun, he fled to Himring and reinforced Maedhros' people. The Feanorians were quite fortunate in the sense that most of the major assaults (even before the breaking of the seige) tended to be focused around Hithlum and the sources of Sirion, including Anfauglith. They may have been able to increase their numbers because of their relative stability, and were probably better prepared to withstand attacks from orcs: they were able to flee to the hills, so Maedhros' people could withstand the volcanic ash that issued from Angband, as well as have the higher ground during the actual battles.