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.

To get free empire points for buying special units and maps (maps for multiplayer and units for help in missons) just go to single player and then empire map.

Make it totally one-sided (i.e., three Franks on the same team vs 1 Briton on another team) but you may choose any map you want (this is what bonus maps are for). Then start the game. If you want a really easy game, turn off black map and fog of war.

Play the game but let the other teams take care of the enemy and just focus on getting as much resources and building the most buildings you can. Once the allies kill the enemy or 15 days pass, the game will end and you will recieve empire points depending on what you did well in.

You will receive no score for military, but economy and others will be really high giving you lots of empire points. Now go back to the main menu and then go to bonus items. Now use these empire points to either buy special units or you may choose new maps for making this easier or for playing multiplayer.

Also, while playing empire maps or multiplayer, if you see some gold in a sack, some broking pillars or a goat, they are not relics, they are extras such as getting the gold in the bag gives you gold, the goat gives you food and the pillars give a variety of things. For the bonuses from broken pillars, use a weak and cheap unit or a unit that is very low in health since that unit may be destroyed when the item container is broken.