The Collaborative International Dictionary of English v.0.48:

Yamen \Ya"men\, n. [Chin. ya a civil or military court + men a
   gate.]
   In China, the official headquarters or residence of a
   mandarin, including court rooms, offices, gardens, prisons,
   etc.; the place where the business of any public department
   is transcated.
   [Webster 1913 Suppl.]