Richie,
In the borders 'castles' can vary in size from a simple pele tower of about 3 storeys (which is probably what is being described here) to the enormous Bamburgh Castle built on a coastal promontory.

Whilst a pele tower was easy to defend against marauders in a hurry the most effective way to attack the occupants was to pile up combustibles around the smaller bottom storey and simply cook the occupants if the smoke didn't choke them first.