Green’s Dictionary of Slang

Holloway castle n.

also the Castle, royal palace of Holloway
[before its mid-20C+ remodelling the gateway of the original Holloway prison (now England’s main women’s prison) had ‘castellated’ architecture, copied in part from Caesar’s Tower, Warwick Castle; the pub across the road is still called the Holloway Castle]

[late 19C–1930s] Holloway prison.