you could just have artilery fire preeptively on defense first round in land and amphibious battles
or artilery defend preemptively on the first round when defending victory cites and amphibious invaisons
but that might get too powerful
People buy infantry as cheap defense and wound fodder
people buy artilery to give their stacks a bigger punch
people buy tanks for their speed and superior attack capiblities
artilery deserves a second reason to purchase it, because it falls between tanks an ifantry in combat capiblities and cost, and already has a puropse on attack, making its defense better rounds it out as a great unit.
my biggest fear is a german player putting 7 artilery in france, and making it untouchable, but then again thats semi-realistic, the bigger you make the atlantic wall, the harder it is to break
and im not a fan of blockhouses, its just not a fun unit. and armies have divisons and corps of men, tanks planes, etc….not divisons of bunkers and forts, those come with the territory and have to be filled with men.
Long story short…make arty more defensive…using the simplest method possible