One of the biggest weaknesses of Mongols imo is that their vills are very exposed and easy to kill. You can make many towers around your base but they do not have a lot of health. In feudal and castle age, knights can raid your base, destroy your towers like it’s nothing and not die from all the arrows even if you garrisson all your vills.
Suggestion:
Make the Pax Mongolica upgrade available in Castle age. This sounds like a fair supportive defensive mechanism for a civ that doesn’t have walls and that is very exposed to raids (ironically).
This upgrade is currently available in Imperial Age, but it’s highly likely that you don’t make it to Imperial Age without having decent defensive mechanisms.