The Roundhouse.

The Roundhouse at the entrance to Aberdeen Harbour is one of the most recognised landmarks in the city. It served as the control centre for harbour navigation for two centuries. Red and green lanterns once used to control shipping are visible on the gantry. Their replacements also can be see on the new modern Marine Operations Centre on the left behind the Roundhouse. This replacement centre was opened in 2006 by Aberdeen Harbour Board - Britain's oldest business.
This building houses the vessel traffic control centre which is responsible for more than 8,000 vessel arrivals a year.
Light signals like those on the Roundhouse are still in use today.
Red : No Entry for ships proceeding to sea.
Green : No Entry for vessels proceeding towards the harbour.
Red & Green : The port is closed.
Best in zoom.

No 2 son used to work in the Roundhouse as Navigation Officer.