Parts of this erstwhile official residence of the chief executive of Hong Kong, and previously the colonial governors, date to 1855. Other features were added by the Japanese, who used it as military headquarters during the occupation of Hong Kong in WWII. It has a public Open Day once a year, one Sunday in March when the azaleas in the gardens are in full bloom.
At other times you can't see much because of the high wall surrounding it, though the security guard will usually let you peep through the gate.