Jaswant Singh Chail, 21, got into the grounds of the castle and threatened to kill the monarch.
In addition to being detained for nine years as the high-security Broadmoor psychiatric hospital, he will be released on licence for five years after serving his sentence.
Following his arrest, he was treated for a psychotic illness and diagnosed as “psychotic, delusional and hallucinating”, Justice Hilliard told the Old Bailey on Thursday.
Chail was said to have been motivated by a sense of injustice and the 1919 Jallianwala Bagh Massacre at Amritsar in which hundreds of people were killed.
Mr Justice Hilliard referred to conflicting psychiatric evidence heard over six days about Chail’s mental state.
