What do the following classic Doctor Who stories have in common?

Warriors’ Gate
The Power of Kroll
The Pyramids of Mars
Planet of Evil
Revenge of the Cybermen
The Mutants
The Abominable Snowmen
The Moonbase
The Smugglers
The Rescue

and, in its own special way, The Edge of Destruction

(Plus, I would argue, The Deadly Assassin, though technically it doesn’t quite fit my categorisation.)