There are two gun emplacements you can freely clamber around plus other installations.
More information about the site and what is there.
What I found interesting was that it was started twenty years before WW1.
I know other forts in Australia were commenced earlier due to concern about Russian expansion. My guess is the German colonies in the Pacific spurred on this upgrade to Western Australia's defences.
Princess Royal was in operation during WW2 but its guns were never used in anger.