Does anyone guard Mount Olympus in mythology or was it sort of just "gods live up there, we stay down here?"
Typically the gods themselves guard it with their own great power, though it is partly guarded just by the difficult nature of climbing a giant mountain.
Two notable attempts to storm Olympus are Bellerophon, the rider of Pegasus, and the Aloadae, Otus and Ephialtes.
Bellerophon just got real cocky and tried to ride Pegasus up there and got struck down by one of Zeus’s lightning bolts, or in some versions, Zeus sent a gadfly to sting Pegasus and make him buck and throw Bellerophon back to Earth, where he landed in a thorn bush, blinding and crippling him for life.
Otus and Ephialtes planned to get to the top of Olympus by stacking other mountains on top of each other to reach the top. Their goal was to kidnap Artemis and Hera and make them their wives. They managed to grab Ares and contain him in a bronze jar for a year, but were ultimately killed by Apollo, and their shades were chained to columns with snakes in the Underworld.