The Collaborative International Dictionary of English v.0.48:

Marshalsea \Mar"shal*sea\, n. [Marshal + OE. se a seat. See
   See a seat.]
   The court or seat of a marshal; hence, the prison in
   Southwark, belonging to the marshal of the king's household.
   [Eng.]
   [1913 Webster]

   Court of Marshalsea, a court formerly held before the
      steward and marshal of the king's house to administer
      justice between the king's domestic servants.
      --Blackstone.
      [1913 Webster]

Bouvier's Law Dictionary, Revised 6th Ed (1856):

MARSHALSEA, English law. The name of a prison belonging to the court of the
king's bench.