The bastion is a battleship-size warship built from heavy frames, heavy steel plate, thrusters and circuits. It has more hardpoints, the mounts that take weapons, than any smaller warship. Unlike the flagship, it is not a command post.
The bastion is locked behind research. An ARCHON unlocks it by trialling a warden design of its own that wears more slowly than the blueprint. A Shipyard then builds it.