The Collaborative International Dictionary of English v.0.48:

Tower \Tow"er\, v. i. [imp. & p. p. towered; p. pr. & vb. n.
   towering.]
   To rise and overtop other objects; to be lofty or very high;
   hence, to soar.
   [1913 Webster]

         On the other side an high rock towered still.
                                                  --Spenser.
   [1913 Webster]

         My lord protector's hawks do tower so well. --Shak.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Towering \Tow"er*ing\, a.
   1. Very high; elevated; rising aloft; as, a towering height.
      --Pope.
      [1913 Webster]

   2. Hence, extreme; violent; surpassing.
      [1913 Webster]

            A man agitated by a towering passion. --Sir W.
                                                  Scott.
      [1913 Webster]

WordNet (r) 3.0 (2006):

towering
    adj 1: of imposing height; especially standing out above others;
           "an eminent peak"; "lofty mountains"; "the soaring spires
           of the cathedral"; "towering icebergs" [syn: eminent,
           lofty, soaring, towering]

Moby Thesaurus II by Grady Ward, 1.0:

128 Moby Thesaurus words for "towering":
   Atlantean, Brobdingnagian, Cyclopean, Gargantuan, Herculean,
   Homeric, Olympian, abysmal, aerial, airy, altitudinous, ascending,
   aspiring, astronomic, august, burning, colossal, consuming,
   cracking, crashing, distinguished, dizzy, dominating, elephantine,
   elevated, eminent, enormous, epic, ethereal, exalted, excessive,
   exorbitant, extraordinary, extravagant, extreme, famous, fantastic,
   fiery, giant, giantlike, gigantic, glorious, godlike, great,
   haughty, heroic, high, high-pitched, high-reaching, high-set,
   high-up, huge, immense, immoderate, imposing, impressive,
   incomparable, infinite, inordinate, intemperate, intense, jumbo,
   lauded, lofty, magnanimous, magnificent, majestic, mammoth,
   massive, mighty, mind-blowing, monster, monstrous, monumental,
   mortal, mountainous, mounting, noble, outstanding, outtopping,
   overlooking, overpowering, overtopping, overwhelming, paramount,
   passionate, preeminent, prodigious, profound, prominent, renowned,
   sky-high, skyscraping, soaring, spiring, steep, stupendous,
   sublime, superb, superior, superlative, supernal, supreme,
   surpassing, tall, titanic, topless, toplofty, topping, towery,
   transcendent, tremendous, ultimate, unconscionable, undue,
   unequally, unmatchable, unmatched, unmeasurable, unparalleled,
   unrestrained, unsurpassable, unsurpassed, uplifted, upreared, vast,
   vehement, violent