Citadel

"That is.. the Citadel..."

- The Doctor to Clara Oswald first seeing the Mega-Structure

The Citadel is an planetary mega-structure created by the Operator 600 years after he fled from Gallifrey.

The Citadel once served as the home of the Operator. Due to the size of the structure, he was able to make any facilities, objects, and even organic material he wanted. Since his TARDIS is the "heart" of the Citadel, it is also capable of time travel. Using Citadel's barriers which protects the structure when time travelling across the time regions.

The Operator also had constructed to be able to make another planetary mega-structures called the Nexus.

Whenever the Operator is not at the Citadel, he had created robots called Dankebot to overseer the structure. Mainly an specialize unit of Dankebot is created, and is given an Greek Alphabet name woth 3 numbers. The "Numbers" represents creation order. There has been over 1000 Dankebots that had been created.

Cooling Systems
Due to the immense heat exerted from the Nexus Construction Facilities, the cooling system was created all over the Citadel, keeping cool during an construction of an Nexus. Due to this also, the cooling systems, the area around the systems alters climate to be snowy, more colder then other parts of the Citadel.