More than 40,000 people are expected to flock to see Ulster's hidden architectural gems this weekend in the European Heritage Open Days.
Among the historic buildings throwing open their doors to the public for free are the Harland and Wolff headquarters, St Anne's Cathedral, Stormont Castle, Crumlin Road Jail, a model village, windmills and a lighthouse keeper's cottage in Whitehead.
Many of the 215 buildings taking part in the two-day culture festival are not normally open to the public - and there will also be a series of events celebrating Northern Ireland's built heritage.
There's even a treasure hunt in the Co Down village of Killough, a Conservation Area, as well as a Titanic Tour in Belfast.
Environment Minister Arlene Foster launched the event yesterday at Harland and Wolff's historic drawing office, where the Titanic was designed.
"I am very grateful to the owners of these buildings who open their doors to provide us all with a rare opportunity to see inside," she said.
"Last year over 40,000 people took part in the organised events and I do hope that those of you who have participated in previous years will make a return visit and for those of you who have not yet participated I strongly recommend doing so and making this 10th anniversary the most successful event yet.
"There are fine examples of buildings ranging from courthouses and jails to a model village; cathedrals to a lighthouse keeper's cottage and a traditional thatched building. Most of these properties are not normally accessible to the public.
"This weekend affords us all a superb opportunity to see what is behind the doors of these wonderful properties and there is something for everyone, no matter what age or interest."