A lot of the words used here are borrowed. For instance 'ethnoi' is what the pre Abrahmic people of the Fertile Crescent called their nation. But I see your point. You should also concede that it actually makes sense for a prefecture to be named after the people or vice versa. RP-wise, [i]sendai[/i] in the Old Tongue means Children. Another interpretation, the more modern one, is that it means the Chosen.

About the tech level: I would say pre-medieval. Since the...ahem...Sendai are forbidden to use their powers to reshape nature, people use conventional methods to force the elements to do their job. Water wheels, wind mills, ox carts, horses, bullock-driven iron or wooden ploughs, extension of the pulley system, decent metallurgy (which is a thriving industry I might add), steel swords, chain mail, no cross bows though. The chain mail was mainly a reaction to the famed longbows of Mineria. And no gunpowder. I am open to both these things appearing in the course of the RP but that's where it will stop. No hand held fire arms certainly.

Architecture. Well, let's see

Mineria - Think Arthurian England, with lots of stone castles and stone houses and clay brick huts in the countryside. Stone is abundant in Mineria. Elband is an exception. It has sandstone from the Gor desert, hand crafted with the power by Altalan the Great. The buildings are taller than anywhere. And sweeping canals across the river Alacra. It is planned city, with plenty of boulevards and courtyards.

Balenia - Pretty much the same as Mineria, as in most cases, but in order to emphasize their differences, they have the habit of using a lot of red bricks. Pretty dreary, I might add.

Gurgan - Since the terrain is fairly hilly to mountainous, there is classic step architecture. Think pastoral Spain. Mostly stone buildings. Nestled amidst olive groves.

Three Kingdoms - A heavily populated region. With lots of big cities full of tight narrow streets and bustling market places. The houses are mostly made of clay and straw. The Three Kingdoms, owing to the constant wars, have the largest urban poor population and a thriving Thieves Guild. Think Crusades Era Palestine.