What kind of castles do you choose?

Well you have to consider that an initial province attack and hold; fort construction and domination of control of provinces for both gem and army production makes a massive difference.

If you could make double the money and production from turn 4 to turn 12 (Just for the early game expansion) then youll have a massive advantage in any field you choose as you have much more options to spend your valuable gold on.

Take an example of Pythium.

If you could build an Extra Temple that pumps A communicant every turn from an initial conquest of 2 provinces before a player playing a 20 Admin province, you suddenly have yourself a drastic advantage when armies collide because of abusive Communions.