I think the King is a great idea, but could use a little flair. My thoughts:
When you purchase the king, you get a prompt similar to the Age Up prompt that allows you to choose one of three traits for the king: 1. Warrior King, 2. Wise King, and 3. Corrupt King. Warrior King gives some sort of military bonus, Wise King gives maybe a research speed bonus, and the Corrupt King gives some amount of gold per villager per min. The purpose of the traits is, to add some flavor to the game and could be tweaked in whatever way to not be game-breaking.
Also, how about we give the Abbey of Kings the ability to train Kingsguard units. Either you train a man at arms or knight, and then garrison them in the landmark until they get spit out as a cooler looking, better equipped unit OR the landmark more simply has the ability to train units. I’m thinking 2nd age, you can train some elite footmen that have better stats than the early man at arms, but that cost more and have a longer train time. And Age 3 and onward, you can train some mounted Kingsguard with better stats than the knight but with the appropriate cost and such. The purpose I think the Kingsguard should serve is mostly cosmetic and for a fun bit of flavor. Cosmetically, I would like to see some capes and bucket helmets. The attached sketch is generally what I am thinking of.
Either way, great game and thank you for continuing to improve it!