The Collaborative International Dictionary of English v.0.48:

Hold \Hold\ (h[=o]ld), n.
   1. The act of holding, as in or with the hands or arms; the
      manner of holding, whether firm or loose; seizure; grasp;
      clasp; grip; possession; -- often used with the verbs take
      and lay.
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            Ne have I not twelve pence within mine hold.
                                                  --Chaucer.
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            Thou should'st lay hold upon him.     --B. Jonson.
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            My soul took hold on thee.            --Addison.
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            Take fast hold of instruction.        --Pror. iv.
                                                  13.
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   2. The authority or ground to take or keep; claim.
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            The law hath yet another hold on you. --Shak.
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   3. Binding power and influence.
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            Fear . . . by which God and his laws take the surest
            hold of.                              --Tillotson.
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   4. Something that may be grasped; means of support.
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            If a man be upon an high place without rails or good
            hold, he is ready to fall.            --Bacon.
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   5. A place of confinement; a prison; confinement; custody;
      guard.
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            They . . . put them in hold unto the next day.
                                                  --Acts. iv. 3.
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            King Richard, he is in the mighty hold
            Of Bolingbroke.                       --Shak.
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   6. A place of security; a fortified place; a fort; a castle;
      -- often called a stronghold. --Chaucer.
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            New comers in an ancient hold         --Tennyson.
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   7. (Mus.) A character [thus ?] placed over or under a note or
      rest, and indicating that it is to be prolonged; -- called
      also pause, and corona.
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The Collaborative International Dictionary of English v.0.48:

Stronghold \Strong"hold`\, n.
   A fastness; a fort or fortress; fortfield place; a place of
   security.
   [1913 Webster]

WordNet (r) 3.0 (2006):

stronghold
    n 1: a strongly fortified defensive structure [syn:
         stronghold, fastness]

Moby Thesaurus II by Grady Ward, 1.0:

49 Moby Thesaurus words for "stronghold":
   acropolis, asylum, bastion, beachhead, bird sanctuary, blockhouse,
   bridgehead, bulwark, bunker, castle, citadel, donjon, fasthold,
   fastness, forest preserve, fort, fortification, fortress,
   game preserve, game sanctuary, garrison, garrison house, harbor,
   harbor of refuge, harborage, haven, hold, keep, martello,
   martello tower, mote, motte, peel, peel tower, pillbox, port, post,
   preserve, rath, redoubt, refuge, safe haven, safehold, sanctuary,
   snug harbor, strong point, tower, tower of strength, ward