A galea was a Romansoldier's helmet. Some gladiators, myrmillones, also wore a bronze galea with a face mask and a decoration, often a fish on its crest. The exact form or design of the helmet varied significantly over time, between differing unit types, and also between individual examples - pre-industrial production was by hand – so it is not certain to what degree there was any standardization even under the Roman Empire.
Originally, Roman helmets were influenced by the neighboring Etruscans, people who utilised the "Nasua" type helmets. The Greeks in the south also influenced Roman design in the early history of Rome. For instance, the ancestor of the Chalcidian helmet, the Attic helmet, was widely used by officers until the end of the empire. Lastly, the Gauls were the peoples who most impacted the design of the Roman helmet hence the popular "Imperial Gallic" type helmets. In addition to this, it is commonly thought that the Gauls also introduced chainmail to the Romans.
The primary evidence is scattered archaeological finds, which are often damaged or incomplete. There are similarities of form and function between them.
H. Russell Robinson in his book The Armour of Imperial Rome, published in 1975, classified into broad divisions the various forms of helmets that were found. He classified four main types of helmets for heavy infantry (with subcategories named with letters) and 30 different types of cheek guards.
Helmets used by gladiators were quite different from military versions.
^Kennett, Basil. "Romae antiquae notitia; or, The antiquities of Rome". 1792, p.275
^Kausel, Cecilia. "Design & Intuition: Structures, Interiors and the Mind". WIT Press, 2012, p.69-70
^Abdale, Jason. "Four Days in September: The Battle of Teutoburg". Trafford, 2013, p.170
^Kelsy, Francis. "C. Iulii Caesaris Commentarii Rerum Gestarum: Caesar's Commentaries: The Gallic War, Books I-IV, with Selections from Books V-VII and from The Civil War". Allyn and Bacon, 1918, p. 622