When he throws the hook to climb to the tower, the end is caught in the gargoyle, however, when he gets up to the tower, we see the end being caught on the tower ledge.
It is very sporting of Mad Hatter and his goons not to remove Batman's cowl while he is sleeping or strap him to the table they have him on, allowing him to wake up and easily subdue them.
Hatter failed to account for a major flaw in his machine in that the subject could wake up by simply dying in the dream.