Forgecities or furnace cities were giant factories in the southern region of the Imperial Throneworld. Their concrete smokestacks were visible from hundreds of miles away, and they darkened the sky with pollution. They were scattered with smelting pits and vast factory complexes that had giant pistons pumping through their rooftops.
The forgecities produced weaponry and vital supplies for the Throneworld and all its colonies and outposts. Silver planned to attack the largest of these forgecities with a Hypertunnel and use the factories there to build him a fleet of time machines based off of the Doctor's TARDIS. (PROSE: Hope)