That's strange. If I build six or more towers, I only get an amulet for building three towers. That only happens on the Kongregate edition, but the 6 tower amulet isn't marked as Armor Games-only.
@PureMusic and all those others who ask why Gemcraft isn't as great as chapter 0. Gemcraft is the original game, released 2008 according to Wikipedia, although I could have sworn I played it years ago. Chapter 0 is a prequel, released 2009. The really good news is that a sequel, Gemcraft Labyrinth, is due for release at the end of January. It's going to be epic.
 "GemCraft"){kind=icon}