A rotating bar of fire anchored to a block inside castles. Firebars spin continuously at a fixed speed and cannot be destroyed by any means. They are purely an environmental hazard — contact with any fire segment deals damage. Later castles feature faster firebars and pairs spinning in opposite directions.
You cannot defeat a Firebar — it is indestructible. The only option is to time your movement through the gaps in their rotation. Count the rotation speed and sprint through the safe window.