The Cloisters  is a museum located in Fort Tryon Pack near the northern tip of Manhattan Island on a hill overlooking the Hudson River. It is a branch of the Metropolitan Museum of Art built in the 1930s. The Cloisters specializes in exhibiting art and architecture from Medieval Europe - mostly 12th to 15th century.
Dr. Doris Tibbs and Peter Venkman attended a special evening showing of the Genevieve Tapestry at the Cloisters. They arrived minutes before the 7:30 pm unveiling but the event caused the release of the ghost Sir Bruce and all other spirits anchored to paintings and statues. The Cloisters, itself, was transformed into sans Pitie Castle. If the effects weren't reversed, they would be permanent and many priceless pieces of art would be lost.
The Ghostbusters entered the Cloisters and took Doris away from the Special Exhibits Room before trying to capture Sir Bruce. At sunrise, he was captured and the Cloisters returned to normal.