The Garden Room had no features and was used to "calm" the resident at Gatooma care home in Holsworthy in Devon, Bristol Crown Court was told.
The boss and four staff at the Gatooma and Vielstone care home, in Bideford, are accused of a string of offences against seven residents.
They deny charges of false imprisonment
The court heard the partially deaf woman, who had anxiety and behavioural problems, was in Gatooma's Garden Room 60 times and twice overnight in 2010, and 132 times in 2011.
Christine Bates, from Wokingham Borough Council, who was responsible for the woman's overall care, was shown the room by the resident on a visit to Gatooma.
"I thought it would have more than one door," she said.
"I was told it was to calm herself down and that staff would keep an eye on her and make sure she was alright."
Prosecutors previously told the court disabled adults were "imprisoned" in the empty rooms to punish or control them.
Paul Hewitt, 70, managing director of Atlas Project Team Ltd, and employees Lee Farrant, 30, James Lawson, 41, Julie Barlow, 51, and Aaron Jones, 33, all of Devon, deny the charges, said to have happened between 2010 and 2011.
Mr Lawson is charged with eight counts of false imprisonment, Mr Farrant faces six counts and Mr Jones and Ms Barlow are both charged with three counts of the same charge.
The case continues.