behind the walls of ered engrin in the west , where they bent back northwards , melkor built another fortress , as a defence against assault that might come from valinor ; and when he came back to middle-earth , as has been told , he took up his abode in the endless dungeons of angband , the hells of iron , for in the war of the powers the valar , in their haste to overthrow him in his great stronghold of utumno , did not wholly destroy angband nor search out all its deep places .