rip-off (I forgive you, however)
I have played a computer game VERY similar to this. It is called Age of Mythology (Aom), and this game is very similar. This is judged by the population, the accessability of units later in the game, and the fact that your "castle" has an upgrade much like the boiling oil upgrade, which allows building to attack units directly below them. I forgive you, however, for what kind of game isn't ripped off?