dominion spread

[Agrajag]

I'm pretty sure dominion spread spreads in a random direction, so if you park temples and prophets and whatnot in a province next to an enemy castle, there's only a small chance the spread will even go to that castle (because your province also has many other neighbours to spread the dominion to).
Maybe if you surrounded their castle with temples in all surrounding provinces...

[Micah]

It mostly just boils down to dom-kill not being a very good way to take out an opposing nation in most cases. The couple of times I've actually managed to use it as a good tactic were one time with preaching (Against Mictlan, meaning he had incredibly low dom-spread and no ability to preach or spread dominion without a temple) and a massive blood-sacrifice campaign with LA Marignon and their H3 priests against a newish player that didn't notice what I was up to, but was playing LE Ermor and just had too many troops to deal with.