Here are all of the unlockable units. You use empire points that you earn in matches to purchase them. To purchase them, just go to "bonus items" in the main menu.

Effect - Code

Dark Ram - 150 empire points
Dopple- handers - 100 empire points
Genoese Crossbowmen - 100 empire points
Knights of the Round Table - 200 empire points
Mons Meg - 150 empire points
Swiss Pikemen - 100 empire points
War Wolf - 150 empire points
Welsh Bowmen - 100 empire points