Spires are creations of the Forgotten, designed against Wizards. They head towards your Orb, towering over everything and you cannot banish them (similar to low-health Shadows from Gemcraft Labyrinth) so you will automatically lose if one reaches your orb. They have varying but generally high HP and no armor, yet their maximum damage taken per shot is capped (Max/HP), so there is almost no use of Bolt or Barrage spells, but Beam and Poison are especially effective (as is Wake of Eternity). When they die, they destroy all nearby walls. Unlike Shadows (GC2), they move very slowly.
Fortunately, they spawn from edges that are farther away from the orb; but that means that orbs in the dead center of the field will be vulnerable on ALL sides.