Now in its third year, Brisbane Open House threw open the doors to 51 buildings and sites, many parts of which are usually closed to the public, as part of a civic worldwide movement that is slowly gaining momentum.

While many of the 51 buildings could be found on the Queensland heritage register, the open day also featured some of the city's new green-star projects, including One One One Eagle Street.

But much like great-grandparents at a family reunion, it was the oldest of the city's showcases that attracted the most attention; prime among them Queensland's oldest surviving structure, the Windmill Tower at Spring Hill.

Built by convict labour in 1828, the 184-year-old tower can also claim the title of the nation's oldest windmill tower.

The Brisbane lord mayor, Graham Quirk, who headed the last tour of City Hall's restoration before its doors reopen to the public in April next year, said sharing special historic buildings with a city's public helped to cultivate their interest in their home's roots.

''Brisbane is a city that truly values its heritage, and events such as Brisbane Open House encourage communities to reconnect with and be proud of their city's past,'' Mr Quirk said. ''Brisbane Open House attracted almost 18,000 visits across 30 buildings in 2011.

''The growing number of buildings open for the day this year, and the number of people who attend each year, demonstrate that the Brisbane public has a keen interest in seeing and protecting our city's heritage for generations to come.''