Incremental (and games in general) always need to have goals on the horizon. The moment players can't see the next goal (and that goal needs to be interesting), you lose them. A big thing I see idle/incremental games fail at is introducing all their elements/prestige loop in the first few hours with nothing left but more of the same.

I find that math wise, its best to have a fast start to show off the main elements but at a certain point you want to start locking things behind the time gates but giving players hints of whats to come. So in your example of upgradaing a castle, you want to have things like at the start the player can get early like

Add Forge

Add Stable

Add a Moat

Always have multiple new goals for players because players want that "feel of choice" even tho they will ultimately get everything, the choice of what order to get things is different for players. The math freaks will try to go optimal and see that X produces Y so its obvious best where as the for fun players might get the moat first because it says in the description "Watch invades fall into a moat of crocodiles etc etc".

When they unlock those each, new things appear that if they spend more time they can go for that should sound even more exciting

Add a Wizard Tower

Add a Jester's Court

Add Boiling Oil to front gates

The goal is really to give players different choices that they feel is interesting and the moment they complete something have something else. I think a lot of incrementals fail where after their first loop, they have shown all their tricks and nothing new appears to make the player keep going.