Several things come to mind.
First - Make it possible to move walls and buildings outside the area of the castle for rearranging. Example move a wall to see what is there before putting down roads or moving the castle so you can see what is behind it.
Second - Make a fifth class of warriors we can build like legendary warriors using something different like diamonds. Add the Lost Arts to create them and even require that the support troops be built at the same time.
Third - Make it possible for designated people to get statistics on the Fortress without having to manually calculate it.
Fourth - Make it possible for designated league officers to return troops to member's catacombs from the Fortress or desired Beacon to prevent having to write many letters to members because there is no trusted chat built into the game.
Fifth - Add another private chat that can contain only trusted or password protected entry.
Sixth - Add a Marshal and Captain only chat for friendly or unfriendly notices to be passed as desired.
Seventh - Add a tunneling set of warriors that can undermine walls or simply tunnel under them as a way of reducing the effectiveness of walls and introduce a devious way into castles requiring a new breed of warrior for counteracting these warriors. Obviously the Lost Arts and Dragon Art upgrades along with appropriate entries in the Obelisk of Power would be needed as well.