First of all, the latest DLC - Chronicles : Battle for DLC that brought 55 land unit models, 19 naval unit models and 85 building models raised my expectations too high for more unit and building models for the main game. And who else deserved them more than our beloved and nostalgic AOK civs?
Castles - Persians and Franks
Here are two unused castles that are already in the game.
I forgot who posted this. Apparently since LOTW, this castle model is hidden inside game files or something.
Other than these two, I don’t really have good suggestions. Here is an attempt -
Britons = Existing one is fine. Needs a Celts castle and then both will have unique castle.
Celts = Maybe Conwy Castle. I’m not sure if it is an English castle or Celts castle.
(Edit : It is an English Castle. How about Stirling Castle then?)
Byzantines = There are plenty of options.
Chinese = Baimaguan Castle.
Goths = An imagination of Fort Bard.
Mongols = Honestly I have no clue. Maybe an imaginary Karakoram city fort.
Japanese = Existing one.
Teutons = Existing one.
Turks = Alanya Castle.
Saracens = Cairo Citadel. Wiki says existing one has some resemble. But I don’t see it.
Vikings = Castle Rushen?
It’s English, one of Edward Longshanks’s many castles in Wales. Probably better to base it on Irish castles, to tie in with the Celts’ Wonder and voice lines.
This would have to be very imaginary (although I don’t see that as a problem personally) – as far as I can tell, there are no reconstructions of the Ostrogothic fort there, and it’s not known for certain whether there was one. I guess they would have built or used forts similar to Roman ones. If I was designing an imaginary castle for Goths, I would incorporate elements of Visigothic and Ostrogothic palaces (Santa MarĂa del Naranco and the mosaic depiction of Theodoric’s palace in Ravenna):
Vikings are tricky… I think this is a good choice – the keep seems to be sufficiently old, even if other parts of it are too new for Vikings.
I had wondered if they could have a Viking ring fortress – but the height of the walls would have to be exaggerated a lot, and they seem to have had wooden rather than stone fortifications:
Alternatively, one could take the attitude that the (at least early) Normans were Vikings, and base their castle on early Norman keeps in Normandy. By this reasoning, they could even keep their current castle (which looks quite Norman), and Teutons could receive a new one instead.