Castles throughout the Middle Ages went through immense evolution, especially their defensive structures. This is because the defensive capabilities of the castle were its main reason for existing, and the castle needed to keep up with siege-craft technology which was evolving just as rapidly throughout the medieval period. In Late Antiquity, the Romans were masters of building defensive structures within several days while on campaign in Gaul and the British Isles. Such structures included walled barracks for the armies to sleep in and watchtowers known as burgus for defending roads and bridges, precursors to the Scottish pele which encompassed the border ‘twixt Scotland and England during the time of Robert the Bruce.