Green’s Dictionary of Slang

marble arch n.1

(UK milit.) the guard-room.

[UK]‘Army Slang’ in Regiment 11 Apr. 31/1: The guard-room (prisoners’ room) is [...] the ‘net,’ ‘trap,’ ‘clink,’ ‘dust-hole,’ ‘cage,’ ‘digger,’ ‘dog’s-home, ’ ‘marble-arch’.