This is a well-made tower defense game. It does not add anything new to the genre but it is fun and well balanced.
There is no storyline but there is a vague World War 2 theme. There are dozens of levels to play, each with dozens of waves, with each wave taking 1-4 minutes to complete. As you complete levels you earn ugrades to spend on various kinds of turrets. This is casual entertainment at its finest.
My main criticism is that some of the levels, particularly the naval levels, take too long to complete. If your turrets just sank the first super-slow but super-tough battleship, then the next dozen such battleships will surely meet the same fate. Yes, you can speed up the action, but there is a problem with repetition.
But this is a minor gripe which is fully addressed in the next game of the series, iBomber Defense Pacific. This game is quite inexpensive and definitely worth checking out if you like tower defense games!