While Melkor was still held captive in Valinor, long before the rising of the Sun or Moon, the Dwarves first appeared in the Blue Mountains. These Dwarves seem to have belonged to two different clans, the Firebeards and the Broadbeams, and each of these founded their own city, on either side of Mount Dolmed in the middle of the range. The northern of these two cities (apparently founded by the Broadbeams - see note 1 below) was named Gabilgathol in the Dwarvish tongue.

The name Gabilgathol means 'great fortress' (compare, for example, Gabilán - 'great river' - a Dwarvish name for the River Gelion). It was translated into Elvish as Belegost, and by that name it was much better known in the histories of Beleriand. In the tongues of Men it would later become known as Mickleburg.

The Dwarves of Gabilgathol often aided the Elves of Beleriand. It was they who helped Thingol create his Thousand Caves of Menegroth, and long afterwards they fought beside the Elves in their Wars against Morgoth. The most renowned of the Dwarves of Gabilgathol was their lord Azaghâl, who wounded Glaurung during the Nirnaeth Arnoediad.