An Energon Tower is an immense construct, even by Transformers standards, that is used to mine, refine, and store Energon. Some are also equipped with shield generators, allowing them to create a dome of energy that can deflect energy and objects, though they do have limits.
Throughout the Energon comics and anime, the Autobots and their new allies, the Decepticons (gasp!) were trying to build these all over Earth while fighting off Terrorcons. After a while, though, Megatron returned, the Cons turned evil, and they started trying to steal the Energon. Of course, this all played into Unicron's continental-size hands. While Alpha Quintesson tried to rebuild the universe, and Megatron tried to recreate it in his own image, the Chaos Bringer was sapping away the energies they were retrieving, slowly rebuilding himself so he could continue his reign of terror. But when the positive energon from Earth stored in his head came in contact with the negative energon mined by Alpha Q in his body, he promptly 'sploded.