Defending the own land has always been a lot easier for several reasons. I like how the designers gave several bonuses to the defender, all adding up to an interesting whole. Cities defending themselves and Oligarchy come to my mind.
It would be boring, one-dimensional, if an empire with 15 archers would just always defeat an empire with 10 archers, all other things being equal. Using defensive strategies will keep you in the game, in case the map generator wasn't clement to you. Furthermore, to achieve a cultural victory, you need to survive long enough against aggressive military/technological players.