" But they gagged his daughter and bound her to the foot of her narrow bed;Two of them knelt at her casement, with muskets at their side!"
The Highwayman is coming.
"Tlot-tlot; tlot-tlot! Had they heard it? The horse-hoofsringing clear;Tlot-tlot, tlot-tlot, in the distance? Were they deaf that they didnot hear?Down the ribbon of moonlight, over the brow of the hill,The highwayman came riding,Riding, riding! "
The Highwayman is trying to get away.
"in the moonlight and warned him - with her death.He turned; he spurred to the West;"
The context is suggesting/over showing that they made sure is was hard for Bess to escape.
Its trying to create suspense for the reader and them to wonder what would happen next.
Its trying to describe the Highwayman's getaway and to show how fast he responded in getting away.