In the mists of time, long before the awakening of Elves or Men, the Valar dwelt east of the Sea in Middle-earth. This was a time called the Spring of Arda, when Arda was lit by the two Lamps of the Valar, and Almaren lay in the central regions of the World, where the light of the two Lamps mingled.
Almaren was covered with grass, and the Valar had a dwelling there, which presumably constituted a city, or at least a town. In the history of the Valar, Almaren was known as the place where Tulkas wed Nessa.
Of much greater importance, though, is the fact that Almaren was the site of the Valar's first major defeat at the hands of Melkor: he loosed his forces secretly from his northern fortress of Utumno (then newly delved). Both the Lamps of the Valar were thrown down and destroyed, and Almaren lay in ruins. Tulkas gave chase, but Melkor escaped into the dungeons of Utumno.