Addictive, different and fun. What more could you want?
Before the game starts you get to choose which traps, spells and weapons you want to use up to a certain maximum. Then you inspect the level to determine where the Orcs will enter and the position of the rift you have to defend. You place traps as you please to kill as many Orcs as possible before they can reach the rift. Then you press "G" to start the game. During the game you earn money depending on how well you play which can be used to place further traps. Sometimes the game pauses between waves, allowing you to set extra traps during the level without the Orcs hassling you. You get extra traps and spells etc as the game progresses and options to upgrade them.
Since I've never played this type of tower-defence-first-person- shooter hybrid before the novelty of it also appealed to me.
Naturally, later levels become harder, with flying enemies and stronger enemies. Sometimes enemies will unexpectedly burst in from new locations or you may have two rifts to defend instead of one.
The game is quite short (6 to 8 hours) and I personally found it very easy. In fact, I only had to restart a level once This didn't bother me, however, because I like easy, short games.
The strategies for success seemed fairly obvious to me but I won't spoil it by saying what they are