> The story is that if you were being chased up the stairs your attacker would trip over onto the roof and you could easily dispatch him

Wouldn't it be equally as likely that while running away in panic, you'd trip on the top stair yourself?

In my opinion, a more realistic explanation is that on plan the stairs were all the same height. But when the builders reached the top, they realised that the roof was an inch higher than planned and just made the final step a bit taller to make the difference?