I’ve been able to develop relatively simple rules to give defenders of an invasion more options.
I’ve added fortified points for each nation (besides fortified cities) in the form of forts (ancient era) and castles (medieval era).
Holding forts/castles affects income collection and provincial control.
Defenders in forts/castles will be able to harass invaders, divert invading forces, or join the main defending army.
Aggressor armies will have the opportunity to ignore, blockade or besiege forts/castles.