When my children were young, I would take them to a nearby park to play on The Castle.
This 'castle' was a clump of large bushes in which a child would hide, about three by nine yards and on a plateau raised about eighteen inches above the surrounding grass.
It is one of the many delights of having a small child to be able to recapture the mind-set which sees this as a castle; and regards visiting such a place as a treat.
The child's imagination does the job, with only the barest of stimuli; yet of course a child is not wholly satisfied by fantasy play. He yearns for the play to be 'real'. He yearns to have real fights against real baddies in a real castle.
However, if he was fighting (and beating) real baddies in real fights in a real castle - he would discover that this did not satisfy him either. It would not fill the yearning.
Indeed, if he had enough opportunities, the child would discover that nothing conceivable on earth can fulfill that yearning to fight real baddies in a real castle.
CS Lewis regarded this as a proof of Heaven, and called it the argument from desire; on the basis that such a desire would not be universal to Men yet impossible unless there was a real place but not on earth where such a desire could be satisfied.
As with all such arguments, it does not compel belief - and any desire can be 'explained away' by some evolutionary or otherwise mechanistic and meaningless just-so story; but I personally find it convincing.