Spires of woe are a type of defense tower built by the Burning Legion. They were used during the defense of Mardum, the Shattered Abyss against the Illidari assault, and again a decade later at the Battle for the Broken Shore.
While at the Broken Shore, they were associated to crystals which anchored the structures to Azeroth. When the crystals were destroyed, the whole structures crumbled. Destroying the crystals associated to the spires in Mardum instead just disabled the structures.