Why not do short text tips like you get during a loading screen in a video game. Would be more likely to be consumed than people watching a video, and allows mobile players on data limits who can't watch videos to learn things. They can just be basic "did you know" type things.

Example : Tacticians only need to send one thief for 100% accurate intel

Example: You can't attack if your population is 115% larger than your maximum population

Etc