The game was made on the fly and quickly, here is something I think could be totally revised to make castles quite interesting.
In AOE3 we had home cities and could modify the city with various unlocks.
It would be really sweet that castles could have this customization for each player too.
Note: this is not to be a pay for thing, but just something to entice and attract players and keep them coming back
This could work in a few ways.
Pure aesthetic- where by rank more options become available such as higher walls, a larger turret, adding wreaths, or custom emblem shield, stone color, making them fancier or more like a fortress.
Where castles impact the game- Some castles were high walled and thick walled. One could choose which castle style they like and use their home city to adopt that castle type. A high wall castle might have a little extra range and LOS, but lose HP, a thick walled castle would have higher HP, maybe cost more and longer build time, With various options to curtail how the castle worked, its built time, attack, ability to garrison units( like walls) etc… would all be slight bonuses or handicaps what would be playable in the game. It would also effect the cost. So someone with a basic castle is going to be able to make more of them, but someone who chooses a really strong castle with a variety of options might pay 2x as much. But remember for each option would also be a trade off too.
I would like to see this for more variety in aesthetics and strategic use of castles.
Just some thoughts, i know a total long shot never gonna happen thing, but its not to late to rethink these things and add fun improvements along the way (as DLCs come out)
give this post a like to show the DEVS this would be a neat feature.