The heater shield was used by almost every class of society in medieval Europe, from knights to typical soldiers. This design lent itself to being relatively inexpensive and easy to make. It was relatively lightweight compared to other similar shields at the time such as the kite shield, being easy to move around during both mounted and on-foot combat, and had a fairly high amount of surface area, making for a solid defense. 

-- From Wikipedia.

The Fire and Steel Teutonic knights crusader order heater shield is constructed for actual use. 60cm x 45cm. Entire shield is steel plate and brass. This is a replica of the shields used specifically by the Knights Templar crusader order of Teutonic knights, who insisted on using a Latin-style cross on their armaments.