Dark Towers (ダークタワー Dāku Tawā, Dub: Control Spire) are large black obelisk structures that were constructed throughout the Digital World by the Digimon Kaiser to block evolution and power Evil Rings.
A Dark Tower from Digimon Adventures 02.
The Digimon Kaiser used hundreds of Dark Towers to both control many digimon with Evil Rings, block evolution and to represent claimed territory. Once destroyed, their effects are negated.
Archnemon has the ability to convert Dark Towers into artificial digimon. The more towers that go into its creation, the more powerful it is. Black War Greymon was created from one hundred towers.