Officers from Thames Valley Police and the Metropolitan Police responded to a security breach at around 8.30am on Saturday, 25 December within the grounds of Windsor Castle.
A 19-year-old man from Southampton was arrested on suspicion of breach or trespass of a protected site and possession of an offensive weapon.
Security processes were triggered within moments of the man entering the grounds and he did not enter any buildings.
Following a search of the man, a crossbow was recovered.
The man was taken into custody and has undergone a mental health assessment – he has since been sectioned under the Mental Health Act and remains in the care of medical professionals.
Enquiries into the full circumstances of this incident are being progressed by Metropolitan Police Specialist Operations.