Dame Anne told BBC Radio 4: "You had a volatile, sometimes vulnerable population, too many staff who were inexperienced and untrained and often didn't want to deal with the population and unsuitable accommodation.
"Put those three things together and you had what we described as a seriously unsafe and poorly controlled environment. "These young people had been put in too quickly without sufficient systems in place and certainly without staff who were confident to be able to operate them."
In their report, inspectors noted: "We came across a number of young people on each unit who remained in their cells... because they said they were being bullied or feared being assaulted or getting into a fight."
Inspections also showed there were high levels of force used by staff to stop inmates, aged 15 to 18, from fighting.
There had also been "inaccurate" recordings of injuries caused when inmates were restrained, the report said.
Paul Carroll, Prison Service South East area manager, said: "Following the inspection, an action plan was immediately put in place to address some of the issues raised.
"I don't wholly accept that the prison was unsafe but I do accept that some boys were challenging in their behaviour.
"The transition from a women's prison to a prison for young people is some of the most challenging work we do."
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