When installed in 1879 the "Military Top" was mounted high up the mast. Later (probably in 1892-3) the "Top" was placed lower down.

"Hitherto, there has been no place aloft from which a "look-out" could be kept, but Captain Mandeville has obtained permission from the Government to step an iron mast, with a circular "top", or platform.
On this will be placed a Gatling gun, which has been ordered from England, and the electric light will also be shown from this "top" at night time for torpedo work. This mast if fitted with a derrick will be found useful in many ways."Argus May 13 1878

Whether the Top on Cerberus ever contained a searchlight or machine gun is not known. The sketch is of a British "Fighting Top" from Steve Johnson's Website.