Hi folks - new to the forum (have put a hello post in the intro section), but as I'm very interested in Border History and the Reivers, thought this might be a good place to start! From my understanding of the way these towers were built and the tactics employed, the basic thought seems to have been 'pull the ladder in and hope they get bored with trying to get in'. I don't think that they would have withstood a major seige, but borderers were more concerned with easier pickings were they could find them, and the cattle / sheep etc were certainly those. They did however employ a tactic known as 'scumfishing', whereby they basically went onto the roof and tried to prise the stone tiles off, therefore giving them access to the upper story.
It wasn't unknown for a family, if they had prior warning of a raid that might involve this sort of thing, to pack the tower full of peat and set light to it. Basically it would render the structure unusable to the raiders, but when they'd gone the family could come back, let the fire burn out or put it out, and then clean up and live there again!
Ah, the 'reivin times, they are a'callin...