A castle is a fortification designed to keep and enemy out, while a prison is a fortification designed to keep an enemy in. But often, and maybe even always, they are basically the same thing. And both are like our beliefs.
In a sense, everyone is the king of their own castle of ideas, but religion takes this to a higher level by encouraging a person to trust their imagination when it tells them they are doing it, not for themself, but for the glory of God.
By believing that their "beliefs" are necessary to survive, have meaning, or even control their own appetites and desires, people are encouraged to become ever more dependent upon the God-drug that all organized religions push. And like the man selling heroin, so religion sells holiness, and both trigger the same release of endorphins in the brain that make a person feel warmly loved.
The only difference is that one of these drugs is illegal and the other is legal. And it's not only legal, it's even encouraged as absolutely necessary, to save oneself from the eternal fires of a Merciful Loving God. A God, mind you, who slaughtered his own son (using humanity as the all too willing knife) like Abraham, in order to forgive it for being stained with original sin - a stain He just happened to withhold from the soul of his own son, so that son could grow up to be the kind of a man God hoped humanity would eventually kill.
And to prove that God should never doubt His faith in man's miserable sinful nature, we were only too happy to oblige.
This narrative of a God forgiving his sinful creation through the sin of them murdering his own son, is used to convince people to doubt everything but those things that affirm this narrative. And by doing so, their dependence upon it to being not just a "belief," but the very bedrock of all discerning wisdom, knowledge, and truth, only grows. And the more this dependence grows, the more it is applauded.
To those believers, their "beliefs" are what are most important, more than anything else in the universe; even more than they are.
To die defending those beliefs (i.e. their "castle") in this temporary universe - which would be smaller than an atom and only amount to a mere millisecond to an "infinite" and "eternal" God - is something that they believe is the highest virtue and honor they can perform. And they believe this not only because it is the surest way to go directly to heaven, without having to pass purgatory, but to show their gratitude to a God who was generous enough to have created the entire universe -even with all of its difficulties, cruelties, and death - specifically for them.
This castle can also feel exactly like a prison to the "believer," however, since it tends to be seen not as a castle at all, but a cross. Religion teaches the "believer" to not only conflate "truth" with a "belief," but to identify their own suffering with the suffering of their Messiah. Such a perspective means all of the suffering added to the world from Crusades and Inquisitions to pedophile priests and the Church's attempts to hide it, all somehow served to redeem humanity in the same way Christ's suffering was alleged to. Hence, if all of the Church's victims had simply offered up their suffering to God, whether from torture or rape, think of all of the people that could've gotten out of Purgatory?!
In this way, Christianity encourages a victimization mentality. Since it is the job of the "believer" to evangelize the world, their view of Jesus as the "victim" of a world that rejected God, leads them to see themselves as Jesus (i.e., "rejected by the world") when anyone challenges the "authenticity" and "infallibility" of their claims. And this is especially the case whenever anyone challenges their claims to "Biblical inerrancy," of either the Bible itself or their interpretation of it.
In either situation, whether seen as a castle or a prison, the point is that both are built out of fear, and the desperate need to protect oneself, from either an enemy without or within. And since religion is also relied upon to keep the darker angels of our being imprisoned within ourselves, chained by our "hope" for eternal life, it is a prison that some preach simply because they rely on it to protect the world from themselves.
But when they discover the power of invisibility that their positions grant them as "spiritual leaders," that is, once they discover that playing the lamb is the easiest way for the wolf to hunt the sheep, it becomes only too easy for them to prey upon victims. Perhaps even with the "belief" that they are actually helping them in some way, as hard as that may be to imagine.
The problem is that religious beliefs do not suppress our fearful and darker side, even though it appears to give some people "hope" through the power of denial of our own mortality, it simply re-channels our darker side, by convincing us that when we are doing our worst toward each other, we are being our best for God.