The Oliver Cromwell Society has a website with basic information on the English Civil War. In light of the growing popularity of Warhammer Historicals’ English Civil War miniature wargaming rules set, this could be useful in filling in some gaps players’ knowledge.
History
Internet Medieval Sourcebook
The Internet Medieval Sourcebook is a fantastic resource for miniature wargamers. I’ve had this one in my bookmarks for several years and haven’t even begun to plumb its depths. The section on the Crusades is particularly useful.