The helmet Cassis or Galea never really changed much during the lifetime of the Roman empire. The design was not of Roman origin, but based on helmets the Romans had seen on their expeditions to foreign territory and was based on these designs.

It did not sit completely flat against the legionaires head, but slightly raised at the top and touching the head only at the rim. The purpose of this is to give the soldier the appearance of being a few inches taller and also protecting the top of the head from blows that would otherwise have fractured the skull, causing severe damage. There were two hinged side pieces , connected at the bottom by a leather lace tied in a knot. The purpose of these was to protect the cheekbones and jaw

On the front of the helmet , about 3" above the rim is a protrusion protecteing the forehead. This was designed to prevent sword blows from penetrating the helmet and causing head injuries.

The rear of the helmet had a guard that protected the neck from sword blows and also missiles that rebounded from the shield of the legionaire behind the soldier. It was angled in such a manner so as to protect the neck, and also allow the soldier to raise his head without the guard touching his upper shoulder blade and so impeding his head movements.

Helmets in battle did not have the horsehair plume shown here. This plume was purely for ceremonial purposes.