Decorated helmets fitted with a metal mask in the form of a human face have been found throughout the lands of the Roman empire, and sometimes beyond it. Despite the significant number of examples (whole or in part) surviving from the 1st to the 3rd c. A.D., these helmets remain an enigma to students of the Roman army. They have usually (though not exclusively) been found either at or close to forts garrisoned by cavalry or in hoards or graves containing other military equipment, often cavalry-specific such as horse-chamfrons. Most have therefore been identified as cavalry helmets, and it is widely accepted that many are of the type referred to by the 2nd-c. general Arrian in his treatise Tactica whilst describing cavalry exercises (hippika gymnasia):