The Siege of Angband was begun after the Noldor had their defeat of Morgoth after the Dagor Aglareb.

The Siege lasted near to 400 years of the Sun, but the Elves could never fully encircle Angband and they could not fully capture it, nor the Silmarils.

The Siege of Angband was finally ended during the Dagor Bragollach.
Encyclopedia entry originally written by atalante_star