For real sadists there is the Armageddon spell, which puts every shamen's followers in an arena for the final battle. Acquiring new spells can be done in a number of ways. To gain them permanently you have to raid a vault of knowledge, a rather nice pyramid building, which your shaman alone can worship at. While on the subject of worshiping, there are also totem poles, which will alter the landscape if you worship at them. Braves are your basic followers, they don't fight too well, but can build structures and can be trained into the other types of follower.
Warriors are the best in a close up fight and can take on multiple braves with no difficulty.
Preachers convert enemy followers to your own and also counteract their opposite number amongst the enemy. Fire Warriors provide a healthy dose of ranged damage, but aren't as good as warriors close up. Spies can disguise themselves as enemy braves and infiltrate their villages, where they can engage in arson. You also have access to boats and balloons. Which are handy ways of moving your followers around and when combined with different followers provide new tactics.