Cheryl Carter, Ms Brooks's long-time PA, denies one count of conspiring to pervert the course of justice, along with her former boss, the ex-News of the World (NotW) editor.
Ms Carter's defence counsel Trevor Burke QC said the charge relates to allegations his client and Ms Brooks conspired to remove seven boxes held in News International archives as the NotW was on the verge of closure following phone hacking allegations, in July 2011.
He said: "It's Cheryl Carter's explanation that she was under pressure to remove (notebooks from) the archive facility as it was moving to Enfield."
The defence counsel said Ms Carter denied claims she wanted to destroy the boxes - containing notebooks and keepsakes from Ms Brooks's office during her time at News International, among other items - "to protect her beloved boss", for whom she had spent more than 15 years working.
Giving evidence from the witness box, Ms Carter described how she was "always on call" working for Ms Brooks. Ms Carter, coincidentally a school friend of Ms Brooks's former husband and ex-EastEnders actor Ross Kemp, said: "She worked fast, everything had to be done immediately.
"I was tasked by email and text. Then we had a record, even if she was standing next to me."
She added: "When I left at the end of the day I had the (office) phone diverted through to my mobile."
Ms Carter, of Chelmsford in Essex, and all other defendants deny all charges.
The trial continues.