The Cloister Room is the most important Power Room on a TARDIS, for it is here that a TARDIS's miniature copy of the Eye of Harmony is found. The room is located at bottom of the Transpower segment of the Time Sceptre. Artron energy travels up the Time Sceptre's central shaft to the Transpower Systems and from there onwards to the Control Room Sphere. Not only does the Eye provide energy for the Transpower System, it also balances all of a TARDIS's multiple dimensions. A side effect of this is that the closer one gets to the Cloister Room, the stronger the dimensional "pressure" gets. This "pressure" can have minor effects on the nervous system.

The Cloister Room of a TARDIS can take any appearance but it will always hold a TARDIS's Eye of Harmony. Most of these have what appears to be an open sky.

Other devices found in this room include the Cloister Bell and special telepathic circuits, which are keyed to the biorhythms of the symbiotically linked Time Lord operator. These circuits can allow for some limited communication between the Time Lord operator and his TARDIS. The interior lights will be dimmed when a TARDIS communicates with its operator. These circuits can also act as an information retrieval system, and provide (through Temporal Projection) a direct audiovisual connection with the Time Lord operator. When used in this fashion it relays whatever the operator sees or hears to the Cloister Room. If a TARDIS "dies", the Cloister Room will be the last part of it that disintegrates.