In adventure mode, you control a party of heroes and explore the world as the struggle between Caesar and the Gaul chieftain evolves. You choose what side to support. You can betray your allies of yesterday. Or, you can follow the mysterious druids as they request your assistance. Command strongholds and large armies; take risky missions behind the enemy lines. Find powerful artefacts to support you. Make friends and foes as you try to reveal who are you and who you want to be
I hope that helps :)
What? How can I activat that mode?
According to the manual, you should see it in the main menu of the game :)
edit: from the manual:
Starting a game
Once you have started the game you are presented with the start game screen.
Since this is your first time playing the game you must define your player. If at
some point in the game you decide to rename or delete an existing player that
can also be done in this section.
After you have set your player you can start the Tutorial to get familiar with the
game. The tutorial will teach you how to successfully use the game’s features
and controls to lead your tribe to power .
This is the adventure of one Gallic hero who must lead the Gauls through this
epic tale in which you must defeat the Teuton and Roman forces and save the
Single player mode implies there can be only one single human participant in
the game. By clicking on the button left of the player’s name the player can
define the number of computer opponents (seven at the most), their race, bonus
and difficulty .
Multiplayer mode allows up to eight players (human, computer or both) to
participate in one game. However, in this mode only the person hosting the
game has the power to set the game settings. The remaining participants can
only define their color, race, bonus and team upon joining.
Once everything is set and every human participant has checked the “I’m
ready” box the person hosting can start the game.
Allows you to load saved games and custom made maps and scenarios.
Alternatively, if you are already familiar with the game and wish to create your
own adventures and maps you can get creative using the Game Editor. See Level
Editor section for more details.
Hope that helps :)