Very nice piece of coding! I like this project, especially for the simplicity of the game. I managed to play it without the tutorial, so i guess it is pretty much self explaining.
The only thing that is not very intuitive are the icons used for the units. Maybe you could use less abstract icons.
I agree with the comments that criticize the final stage of a match. At the beginning movement is quick and the AI can surprise you by sneaking in and conquering a city. At the end there are so many units that the game becomes very static. And when I control a certain number of cities the AI has no chance to win the match anymore. It is just a question of time for a player to line up units and conquer the last cities. Maybe you could add an optional short game, where the match is won with the control of less cities. Or make the cities produce less units towards the end of the game.
Does the AI use any artillery? In the matches I have played so far, the AI has never produced artillery. That is a big disadvantage: when I line up my units (1st row cavalry, 2nd row infantry and 3rd row artillery) the AI has no way to break through, as the artillery will always increase my chances to win and simply makes the difference.
Another suggestion: you could consider adding more terrain types: mountains, rivers, swamp and especially roads. The game reminds me of The Perfect General (early 1990s). In that game moving units on roads would increase their speed significantly. That could also solve the reinforcement problems, that were mentioned here (having to move units from remote cities to the front line).
When there are 3 or more players there could be an option to make allies. To keep the game simple you could simply train the AI to stop attacking a player when that player has not attacked the AI for a certain period time. Alliances could bring new tactical options to the game. ... And the AI could sometimes be nasty and attack the player anyway.