I enjoyed Professor Dorsey Armstrong’s Years that Changed History: 1215 course.
One interesting tidbit: When Genghis Khan wanted to take over a big fortified city, he would attack surrounding small villages and drive the inhabitants as migrants and refugees to the big city. Swelled with a larger-than-usual population, the big fortified city would collapse from within, its resources (such as food) exhausted. This saved Temüjin, the Great Khan, the effort of a traditional Greek/Roman-style siege and catapult attack on a walled city.
Thus did Genghis Khan assemble the largest contiguous land empire that the world has ever seen.