It is important to make sure that you have buildings in your deck as they are used to hold the line. Although they cannot move their purpose is to ensure your frontline stays protected. Each building will have a certain number of hit points allotted to it that will enable it to destroy any opponents soldiers attack that comes under it.
A good tactic is to use buildings to buy you more time to get reinforcement into play by placing them on your frontline. Some building offer an added bonus by spawning new soldiers every turn.