A cervelliere is a close fitting cap made of steel or iron, used as a helmet. First introduced during the late 12th century and was the primary head protection for Crusaders at that time. It was worn either alone or more often over or under a mail coif.

Over time, the cervelliere experienced several evolutions. Many helmets became increasingly pointed and the back of the skull cap elongated to cover the neck, thus developing into the bassinet. Cerveillieres were worn throughout the medieval period and even during the Renaissance. They were cheap and easy to produce and thus much used by commoners and non-professional soldiers who could not afford more advanced protection.

Attribution: https://en.m.wikipedia.org/wiki/Cervelliere