By Your Command

An Imperator Titan is among the mightiest of all the Imperium's war machines, a walking behemoth that can tower over the tallest wall, a nighimpregnable bastion, a symbol of the GodEmperor's might. But any bastion can be penetrated, any fortress invaded. And when the Imperator Titan Casus Belli falls victim to the rampages of enemy forces, its defences are activated – such as the Kastelan robots of 6Terror maniple, who must defend the mammoth Godmachine from the Heretic Astartes who would tear it down…

Read it Because
It's a tale of battle inside an Imperator Titan, as Kastelan robots – perhaps the coolest of all the Cult Mechanicus' war machines – fight Chaos Space Marines.