From WoWWiki

The Halls of the Law is located inside Blackrock Depths, and is accessible from the Detention Block. They are the tunnel system that surrounds the Ring of the Law, and clearing them out is a necessity if you want to do Marshal Windsor's escort quest,  [58] Jail Break!.

Subzones of Blackrock Depths (Blackrock Depths regions)
The Black Vault • Chamber of Enchantment • Dark Iron Highway • Detention Block • The Domicile • East Garrison • The Grim Guzzler • Hall of Crafting • Halls of the Law • The Imperial Seat • The Iron Hall • The Lyceum • The Manufactory • The Molten Bridge • Mold Foundry • Ring of the Law • Shadowforge City • Shrine of Thaurissan • Summoners' Tomb • West Garrison