§ Of the Return of the Noldor The Silmarillion

A victory it was, and yet a warning; and the princes took heed of it, and thereafter drew closer their leaguer, and strengthened and ordered their watch, setting the Siege of Angband. which lasted wellnigh four hundred years of the Sun. For a long time after Dagor Aglareb no servant of Morgoth would venture from his gates, for they feared the lords of the Noldor; and Fingolfin boasted that save by treason among themselves Morgoth could never again burst from the leaguer of the Eldar, nor come upon them at unawares. Yet the Noldor could not capture Angband, nor could they regain the Silmarils; and war never wholly ceased in all that time of the Siege, for Morgoth devised new evils, and ever and anon he would make trial of his enemies. Nor could the stronghold of Morgoth be ever wholly encircled: for the Iron Mountains, from whose great curving wall the towers of Thangorodrim were thrust forward, defended it upon either side, and were impassable to the Noldor, because of their snow and ice. Thus in his rear and to the north Morgoth had no foes, and by that way his spies at times went out, and came by devious routes into Beleriand. And desiring above all to sow fear and disunion among the Eldar, he commanded the Orcs to take alive any of them that they could and bring them bound to Angband; and some he so daunted by the terror of his eyes that they needed no chains more, but walked ever in fear of him, doing his will wherever they might be. Thus Morgoth learned much of all that had befallen since the rebellion of Fëanor, and he rejoiced, seeing therein the seed of many dissensions among his foes.