Adding yet more depth is the possibility of researching new technologies. The more time you spend building up your settlement, the more advanced your units can become. You can even progress to Feudal and Imperial ages, which affect both the look of your buildings and how powerful and advanced your military units are.
If it all sounds too complicated, don't worry. Age Of Empires II does a great job of introducing you into the complexities of its world slowly and gently. And there are plenty of game modes for you to investigate until you feel comfortable. Deathmatch and Regicide are straight kill-or-be-killed scenarios for those of you who want instant warfare, while the Campaigns offer a slower, more structured and varied approach. One mission could have you doing anything from searching for hidden relics or rounding up a flock of sheep as a gift for a potential ally.