The Screens of Zolfa-Thura were large metal plates situated on the barren planet, Zolfa-Thura. There were four plates in total that could amplify signals. Meglos made use of a room hidden in the centre of the plates. This room could rise up during the amplification process.
Meglos installed the Dodecahedron atop the room and intended to use amplification to destroy Tigella. The Fourth Doctor reverted the calculation and it destroyed Zolfa-Thura. Prior to their visit, the Doctor and Romana II both knew of the plates and their power. (TV: Meglos)