Sentry Units were stationary robotic guards that protected the Vault. In its battle to escape the Vault, a Dalek was forced to elude numerous Sentry Units as they proved impervious to its gunstick. (NOTVALID: The Last Dalek)
Behind the scenes
The Sentry Units featured exclusively as enemies in the video game The Last Dalek, based on TV: Dalek. They were unable to be destroyed by the player-controlled Dalek but, should they sustain several hits, they would retreat themselves into the ground temporarily. As well as that, they could not rise up as long as the Dalek occupied their space.