The torpedo siege platform was a proposed design developed by the Galactic Republic's Special Weapons Group in the aftermath of the Naboo Crisis of 32 BBY. However, because of the Republic's pacifistic leanings and the consequent lack of funding, the project never made it past the planning stage.
During the Clone Wars, the platform was compared to the proposed Death Star, although Vice Chancellor Mas Amedda considered it to be a mere toy in regards to the colossal scale of the Death Star project.
After the rise of the Galactic Empire, successor government to the Republic, the Rebel Alliance received intelligence of the manufacturing of Torpedo Spheres, which served as siege platforms and were two kilometers in length.
- Catalyst: A Rogue One Novel (First mentioned)