From their website:
Through careful excruciation, training, and the administration of certain drugs, beast handlers craft these titans into the mighty sentries. Their skulls are hard as stone, weapons unto themselves. Armed with shield and halberd, the massive titan sentries can withstand punishing damage before retaliating in a flurry of crushing blows. Those who come too close find themselves quailing before the enormous creatures, barely able to move as the sentry brings its mighty weapons to bear.