https://www.britannica.com/event/Reconquista - Encyclopedia Britannica entry covering various aspects of the Reconquista of Spain/the Iberian Peninsula.
https://www.youtube.com/watch?v=szxPar0BcMo – Crash Course video about the Mongols. The video covers their rise to success and downfall; it also discusses why the Mongolian Empire is so significant in world history.
https://www.youtube.com/watch?v=X0zudTQelzI – Crash Course video about the Crusades. It explains the reasons behind the crusades as well as the impact it had on the regions and nations that were invaded.