Tower defense games review

I Played a quite a few TD games during lunch time recently, some of the games added brilliant twists to this sub-genre, i listed the them here

1) Desktop tower defense - path finding, custom path

2) Xeno Tactic 2 - each tower can only upgrade certain amount of times and each time can only update a type of attributes (range, speed, damage), this created unique attributes for each tower

3) Shock to the system - Flash 3d with tron style graphics

4) GemCraft - the gem crafting system

5) Vector TD - non-tower, general purpose buildings for more damage, range and firing rate. and also low level tower has unique attribute make them valueble even at latter part of game