The Collaborative International Dictionary of English v.0.48:

Acropolis \A*crop"o*lis\, n. [Gr. 'akro`polis; 'a`kros extreme +
   po`lis city.]
   The upper part, or the citadel, of a Grecian city;
   especially, the citadel of Athens.
   [1913 Webster]

WordNet (r) 3.0 (2006):

acropolis
    n 1: the citadel in ancient Greek towns

Moby Thesaurus II by Grady Ward, 1.0:

31 Moby Thesaurus words for "acropolis":
   bastion, beachhead, blockhouse, bridgehead, bunker, castle,
   citadel, donjon, fasthold, fastness, fort, fortress, garrison,
   garrison house, hold, keep, martello, martello tower, mote, motte,
   peel, peel tower, pillbox, post, rath, safehold, strong point,
   stronghold, tower, tower of strength, ward