In the later twelfth century a range of ‘crusader’ helmets became popular. These were generally flat-top helmets either with a nasal or a face plate, worn over an arming cap and coif.
By raising the surface of the helmet, flat-top helmets provided a 'crumple zone' which could be padded to reduce the impact of blows.
It is an option for knights and well-equipped serjeants from the late twelfth century (c.1160) onwards. They may have flowing cloth decoration attached to the top of the helmet.