Level 10 keeps do look bigger on the world map and they have a unique design when you look inside your city.
However, after talking to some experienced players, it seems that is a bad idea to upgrade your keep to level 10 instead of upgrading the Academy or the Great Hall.
Level 10 Great Hall allows you to level your hero to 212. If you have it to level 9, your hero can only reach around level 180.
Academy allows you to do level 10 researches.
Level 10 keep is allowing you to conquer the 10th wild and have three more field resource plots.
And level 10 keeps could trigger too much attention and you might get attacked more often. But depends, new players will fear you :).
What would you upgrade to level 10?