The castle most likely didn't serve as means to control the flow of inbound people (e.g., as a customs facility), but rather as a prison.

From the 15th century, Hospental castle was no longer inhabited and gradually fell into a dilapidated state.

Clearly visible to this day are the so called garderobe, the locations of the fireplaces, as well as artificially created ditch, which serves to protect from enemy approaches.

[4] In season 11, Sam Denby finished his final run here (as an allusion to the castle’s previous appearance on the show) after a string of fortunate train connections.

He won the series at this location after being tagged by Chase and Ben Doyle less than an hour before the end of the game.