The Collaborative International Dictionary of English v.0.48:

entrenchment \entrenchment\ n.
   1. an entrenched fortification; a position protected by
      trenches.

   Syn: intrenchment.
        [WordNet 1.5]

   2. the act or process of entrenching.
      [PJC] entrepot

WordNet (r) 3.0 (2006):

entrenchment
    n 1: an entrenched fortification; a position protected by
         trenches [syn: entrenchment, intrenchment]

Moby Thesaurus II by Grady Ward, 1.0:

74 Moby Thesaurus words for "entrenchment":
   abri, approach trench, aqueduct, bunker, canal, canalization,
   channel, communication trench, confirmation, countermine, coupure,
   crimp, cut, deep-rootedness, deep-seatedness, dike, ditch,
   double sap, dugout, embedment, encroachment, entrance,
   establishment, fire trench, fixation, fixedness, fixity, fixture,
   flying sap, fortified tunnel, fosse, foxhole, gallery, goffer,
   gutter, ha-ha, impingement, implantation, imposition, incursion,
   infiltration, infixion, influx, infringement, injection, inroad,
   insinuation, interference, interjection, interloping,
   interposition, interposure, interruption, intervention, intrusion,
   invasion, inveteracy, irruption, kennel, mine, moat, obtrusion,
   parallel, pleat, sap, slit trench, stabilization, sunk fence,
   trench, trespass, trespassing, trough, tunnel, unlawful entry