In most games castles and fortresses and such are totally worthless, because the enemy has rarely any reason to attack them. And even if he does, he can usually siege them out instead of attacking full on.

Will there be such constructions in the game(seperate of cities) and if so, what kinds of bonuses they give?

My suggestion would be actually pretty similiar to that of Galciv2 Starbases. A castle would offer bonuses to troops in it's radius, in addition to being able to garrison the units there. Perhaps even economic bonuses could be given, as the people nearby feel safer with a castle.

It always seems such a shame that defensive structures very rarely play a meaningful part in TBS games.