Mechanus

Mechanus was a jungle planet that served as the homeworld of the Mechanoids. It was also inhabited by Gubbage Cones, and had terrific metal deposits which the Mechanoids mined.

The planet was protected with the orbital defence grid and was the capital of the Mechanoid Empire. (WC: Planet of the Mechanoids)

History
As Steven Taylor explained, humans sent Mechanoids to prepare Mechanus for colonisation, but Earth got caught up in interplanetary wars such as the Human-Draconian War, and the Second and Third Dalek Wars. No one was bothered with the abandoned Mechanoids, figuring they would run down or wear out, but the Mechanoids constructed their city and repaired themselves, continuing to function (PROSE: The Chase) even though the colonists never arrived. Steven Taylor crashlanded on the planet and was taken prisoner by the Mechanoids for several years, before being saved by the First Doctor and his companions as the Mechanoids and their city were destroyed by the Daleks, while the travellers escaped in a Dalek time machine and the Doctor's TARDIS. (TV: The Chase)

"The Government decided to open up this arm of the Galaxy, and sent out a fleet of ships to terraform any marginal planets. This place was one of them, and the shipful of colonizing robots came down to clear the landing sites, start building and generally make things as perfect as they could for the colonists. […] Earth got into the Draconian conflict, then the Third Dalek War. That ended the population expansions problem pretty drastically, and this arm of the Galaxy was promptly forgotten."

- Steven Taylor

Somewhat contrary to Steven's assessment, however, Jyl Stoker claimed in 2382 that the Mechanoids were outdated long before then, (PROSE: Fear of the Dark) although another source supported Steven's version of events. (PROSE: The Whoniverse)

Shortly after encountering the Mechanoids, the Daleks set the rogue planet Skardal on a collision course with their home world, but the Zerovians' robot agent 2K prevented Skardal from destroying Mechanus. (COMIC: The Rogue Planet, Impasse)

Trying to make themselves look weak to manipulate the Mechanoid Empire, (WC: The Deadly Ally) the Emperor of the Restoration, accompanied by the Dalek Prime Strategist and only a few Silver Daleks, arrived at Mechanus and claimed to want alliance from the Mechanoids against the Entity. The Emperor explained the danger, and Mechanoid Queen offered an alternative solution; let the Daleks fight the Entity themselves. However, the Emperor then revealed he had led the Entity to the planet. When the Entity arrived to the planet, the Battle of Mechanus broke between Mechanoids and the Entity. After the Dalek Strategist and Mechanoid 2150 returned the Entity back to where it came from, the Queen ordered the Dalek Emperor and Strategist to leave Mechanus. Believing that they were the only two Daleks left in the universe, she promised that the Mechanoids would bring an end to the Daleks. (WC: Planet of the Mechanoids, The Deadly Ally) However, the Strategist and Emperor had engineered this outcome, as the Dalek Empire had actually survived the Entity's crusade. Thus, when the Mechanoids left to attack Skaro, they were overwhelmed by the Daleks. (WC: The Deadly Ally)

Behind the scenes

 * According to non-narrative material in DWBIT 3, an account was taken of the Daleks' battle with the Mechanoids during the former's mission to capture the Doctor.
 * In a Christmas ident when a Dalek is booking a flight from an Earth airport Mechanus is noted to be near Skaro.
 * Initial drafts had the planet named Mechon.