Some more info on the kettle-helm. It was used in Europe from the 12th to the late 15th century and was indeed very popular among all different kinds of men-at-arms from foot-soldiers to horse-soldiers. Even knights used them once in a while as they offered good protection and breathing was much easier in comparison to the great helm. It was highly popular in Palestine due to the hot climate.
There are basically two types, sometimes called an English and German version. The English version is round, like the LEGO version, while the German type is more angled and flat on top. Actually both names are silly, as both types were used throughout Europe.
"German" type: http://der-guertler.de/images/macj_helm.gif
Here you can find several different types of helmets, including the different kettle-helmets and the great helm.http://www.manesse.de/BildSuche.php?id=358&s=Helm&a=erg&