From the preview:
For centuries, the bastion has rebuffed attacks by armies of beasts, warriors, and even mages. Its mighty walls have endured without losing so much as a chip from a single stone. Yet never before has the citadel faced such a massive army, organized with one goal: utter destruction. Only the mightiest heroes can hope to defend the fortress against this evil horde.
In Bastion, the dark forces arrayed against you are more than run-of-the-mill orcs and goblins. Instead, you face an army of different creatures, from nimble scouts to stealthy spies. Each enemy makes its own unique contribution to the siege effort and presents its own challenge to the heroes who defend the fortress.