From the games that i have played for my research, i have seen that there are no real surprises within the tower defence genre, we have made sure our game does not follow this pattern, our game now keeps players on their feet and ready for action as we have added random events to the game, firstly we have added a boss spawn. When the boss enters the game all the other enemies stop spawning so you must fight off the boss and his minions, and when done you will get a hefty reward, this mechanic has been met with a lot of positive feedback as players can now play the game over and over and it wont even be the same and boring, as they have random events that will spawn that will make everything a lot harder.
Evolution of Defence 