Booby trap
A booby trap was a secreted device that would attack those who triggered it.
A member of professor Parry's archeological expeditionon on the planet Telos was electrocuted after trying to open the door that lead to supposed Cybermen tomb. The Cybermen, who died out centuries ago, set the booby traps to test the intelligence of those entering their resting place. The Second Doctor helped the expedition to unelectrify the door and avoid other dangers inside the tomb. (TV: The Tomb of the Cybermen)
Knowing there was a chance Eldrad would return to Kastria, the Kastrians placed booby traps in their subterranean dwelling.
When Eldrad did come back, after one hundred and fifty million years, a door he opened triggered a booby trap. A dart containing an acid that neutralises the molecular bonds and eventually will shatter the crystal matrix in his silicon-based body hit him. As there was no antidote, the acid being of Eldrad's own design, he asked the Fourth Doctor to deliver him to a regenerator chamber.
They dodged other traps on the way, including a motion triggered gas in a passageway on the level 306 and some falling rocks.
The door of the chamber was also fitted with a booby trap, which the Doctor detonated with no ill effects. (TV: The Hand of Fear)
