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.
Select the "Bonus Items" option at the main menu to buy the indicated map in Empire Map mode for the corresponding amount of Empire points.
Complete all campaigns with all characters to unlock the Black Market icon at the main menu. This will allow you to buy previously unreleased characters, maps, campaigns, and movie clips.
If you have some villagers and they need to escape, send them to a building in the direction you want them to go. Choose "Demolish" but then select "No". You will be on that building but still unused. If you have buildings in a sort of line they can get far away in one turn.