You can send out Nomads to attack others.

Nomads need to be recruited if you lost some in battle. To do this, you need to click on the Nomads Icon on the left and choose, via the scroll bar, how many Nomads you would like to recruit.

With every upgrade of the Nomads building you will be able to recruit 15 more Nomads with each level. You get 5 nomads per level after level 40.