Previous episodes mentioned: Episode 7: Heroes and Heretics: Joan of Arc; Episode 9: Worship from the Earliest Exile to the Great Ejection.

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Khan Academy video on the spread of Islam

Islam and Europe timeline

Various versions of Urban II's speech at the Council of Clermont

The First Crusade: A New History by Thomas Asbridge (free to borrow from the Internet Archive)

Encyclopaedia Britannica article on the Crusades