NOUN (2)

1. (military) a temporary or supplementary fortification; typically square or polygonal without flanking defenses;

2. an entrenched stronghold or refuge;

The Collaborative International Dictionary of English v.0.48:

Redoubt \Re*doubt"\ (r?*dout"), n. [F. redoute, fem., It.
   ridotto, LL. reductus, literally, a retreat, from L. reductus
   drawn back, retired, p. p. of reducere to lead or draw back;
   cf. F. r['e]duit, also fr. LL. reductus. See Reduce, and
   cf. Reduct, R['e]duit, Ridotto.] (Fort.)
   (a) A small, and usually a roughly constructed, fort or
       outwork of varying shape, commonly erected for a
       temporary purpose, and without flanking defenses, -- used
       esp. in fortifying tops of hills and passes, and
       positions in hostile territory.
   (b) In permanent works, an outwork placed within another
       outwork. See F and i in Illust. of Ravelin. [Written
       also redout.]
       [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Redoubt \Re*doubt"\, v. t. [F. redouter, formerly also spelt
   redoubter; fr. L. pref. re- re- + dubitare to doubt, in LL.,
   to fear. See Doubt.]
   To stand in dread of; to regard with fear; to dread. [R.]
   [1913 Webster]

WordNet (r) 3.0 (2006):

redoubt
    n 1: (military) a temporary or supplementary fortification;
         typically square or polygonal without flanking defenses
    2: an entrenched stronghold or refuge

Moby Thesaurus II by Grady Ward, 1.0:

60 Moby Thesaurus words for "redoubt":
   abatis, advanced work, balistraria, bank, banquette,
   barbed-wire entanglement, barbican, barricade, barrier, bartizan,
   bastion, battlement, breastwork, bulwark, casemate,
   cheval-de-frise, circumvallation, citadel, contravallation,
   counterscarp, curtain, demibastion, dike, drawbridge, earthwork,
   enclosure, entanglement, escarp, escarpment, fastness, fence,
   fieldwork, fortalice, fortification, fortress, glacis, loophole,
   lunette, machicolation, mantelet, merlon, mound, outwork, palisade,
   parados, parapet, portcullis, postern gate, rampart, ravelin,
   redan, sally port, scarp, sconce, stockade, stronghold, tenaille,
   vallation, vallum, work