A padded linen cap was the most basic form of medieval head protection. It was constructed in the same way as the aketon, with a layer of padding, such as flax or horsehair, stuffed in between two layers of material. This formed a tough, dense cushion to help protect the skull of the wearer.
Some arming caps may have had hidden interlinings of mail if the wearer could afford it.
The arming cap could not provide very much protection on its own, but it was certainly better than nothing.
But often the arming cap, like the aketon, formed the base-layer over which other pieces of armour were worn. A mail coif was often worn over it, but the most vital protection was some kind of rigid helmet made of hardened leather or, preferably, iron or steel.
Layering was the secret to the best head protection, building out from the foundation provided by the arming cap.