The Warden was a tentacled creature that lived in a tank that controlled the atmosphere and gravitational levels within an ancient park located on the Moon in 1878. Captain Richard Haliwell and Emily Boyes-Dennison were captured by its robotic servants and forced to take a few daring tests.
The Warden was described as a deliberately stupid creature, only taking action when it was explicitly threatened, such as allowing the ships of the British Imperial Spacefleet to land in the park despite their lack of authorisation even if it wouldn't let them leave afterwards, or letting the Vrall build a basic cannon because it seemed to be no direct threat to the citadel.
After the expedition gained access to the Citadel, the Fifth Doctor and Turlough — the only two able to communicate with it thanks to the TARDIS translation circuit — tried to reason with it to release the gravitational pull and let the ships leave, including the Phiadorans. They soon realised that, while it continued to obey its original orders, whether out of habit or deliberate conditioning, it apparently wanted to die after being abandoned for so long. Carefully wording his requests so that the Warden would display the relevant panels that had to be disabled to shut down the citadel's protections, the Doctor was able to direct the expedition to cut off the power to the citadel's force-field. Whether through damage to its life-support systems or the damage to the citadel rendering its purpose meaningless, the Warden then died, collapsing to the floor of its tank. (PROSE: Imperial Moon)