Bastions are ballistae mounted on Galadrian's Wall.
Two Bastions initially appear in Campaign mode: one is active and fully upgraded, while the other is broken and requires 250 Gold to fix. The first one does not last long, as it is torched by a Gnoll Burner in wave 5, while the other stays during all three modes.
Bastions fire razor stars that stay on the ground for a few seconds, continuing to deal damage to enemies that walk near them. This potentially allows them to deal extremely high area damage and clear foes very fast (especially when combined with a Barracks), but the first Bastion's early destruction and the second one's small coverage does limit their usefulness.
Bastions do not have a turret, meaning they can only fire in a fixed direction.
Sharpened razor stars deal more damage each.
The intact bastion is yours for free for the first five waves, but is then inevitably destroyed by a Gnoll Burner, and so can effectively be ignored tactically; it kills a few mobs of gnolls but is gone before the Blood Ogres appear. The broken bastion costs 250 to repair and can be upgraded twice, so its full cost is 650 gold. It's best to either completely ignore it and spend the gold on other towers or to upgrade it fully and let it provide some defense against enemies coming from the upper entrance. It's useless against Perythons, however.