nice
altough the idea of shooting bad guys, while avoiding good guys is pretty old, i enjoyed this game. esecealy how you fight through time. beated it without using the walkthorugh (which is a realy good walkthourhg, watched it after the game). if you are planing number 3, maybe you should add diferent cannons and explosives for each time. like the cannon used here, should be used for the first levels.
for the war it should be a modern cannon and for the space a sci-fi cannon.