Earlier in the trial, the court heard Windsor, who has been diagnosed with schizophrenia, had sent a note calling Ms Klass a “naughty vixen”.
In giving evidence, she said she was initially not told about the post to avoid worrying her, but was later shown a list of items and photographs of some of the letters.
“It was very clear, very quickly, that it was a highly volatile selection of items,” she told prosecutor Timothy Sapwell.
“It was a huge shock, especially the extent to which it had escalated. It’s a huge violation. It’s sheer terror, really.”