Have all the cats been spayed and neutered? Altering the cats may really cut down on the kitty drama and the need to defend a certain territory. Are you leaving your cat's food out or growing catnip in your yard? These will definitely encourage other cats to head on over and enjoy the spoils. If they aren't actually physically fighting I wouldn't worry to much. My cat hisses and postures all the time to show he's the top cat in the neighborhood, even though he is safely confined. We have a chain link fence with the cat fence-in on top to keep Sable in his own yard to keep other critters out. You can read about it at http://www.catfencein.com/. You could probably make a cheaper version of your own without to much fuss.
Good luck to you! I know how frustrating it can be when dealing with neighbors and their pets.
