The Cave Monsters
The confrontation between the Doctor and The Beast was filmed at Clearwell Caves, previously used as the Sycorax ship in The Christmas Invasion.
It's The Pits!
The last creature the Doctor discovered in a pit was the Tythonian ambassador Erato in, aptly, The Creature From The Pit (1979).
The virtually indestructible TARDIS rarely suffers any damage when falling from great heights. It fell off a mountain in The Curse Of Peladon (1972).
Creatures and Dæmons
The episode suggests a connection between The Beast and the Dæmons of Dæmos, referred to in The Dæmons (1971). Russell T Davies later claimed that the aim had been to create a "Russian doll" effect, wrapping this episode's mythology around that of The Dæmons.
Claire Rushbrook went on to appear in a special Doctor Who edition of quiz show The Weakest Link, which was won by Camille Coduri.
Beam Me Up!
The TARDIS has previously used a form of tractor beam to tow a spacecraft in Delta And The Bannermen (1987) and hold a neutron star in Creature From The Pit (1979).
Stop Making Sense
Russell T Davies intended for the Ood to resemble the Sensorites, and that he liked to think that they came from a planet near the Sense Sphere. This was later confirmed in Series Four's Planet of the Ood.
Balls Of Steel
Before settling on The Ood, the creatures that would eventually become The Toclafane were considered as the monsters for this story. The Toclafane were originally created as a potential replacement for the Daleks in Series One, when negotiating rights to use the latter hit a sticky patch.