Suggestion: Get rid of Star Forts, or do something to make them not completely useless.
The same goes for bunker and nuclear shelter, too, in my experience. For similar reasons; if you need them you are playing a lost game. Though Star Fort is more obvious.
I also seldom need walls and castles either, although they see much more uses if you have aggressive neighbors.
Ideas...
Don't castles give +1 culture with heritage? If not, make it so. Another +1 for castles with Star fort present, another +1 for a castle and Museum combo. A +1 culture for the castle+monastery combo might be over the top, though great.
On another hand, the (old, tighter) city walls were historically an economical problem, so maybe they could receive a malus (-1 commerce in the city plot?) with discovering (macro)economics or with their obsoletion tech. Then, a (wider) Star fort can offset that malus by removing the walls? (same effect with Park, it removes old pesky walls!)
Or the Star fort could get the defense bonus that the walls have, but only against gun powder and cavalry units. If they currently protect against bombardment through air units, that should be removed though, imo. (no idea if that's an issue, I never was attacked by bombers. Ever. I rarely even use them myself because of their high costs and tiny operation range.)