The Gask Ridge system is a series of forts, fortlets, and timber watchtowers situated along a Roman road in northern Scotland. The high intervisibility of the watchtowers in the Gask Ridge system has resulted in the proposal of two main functions: one that the watchtowers were a signalling system and two that the watchtowers provided visibility of the Gask Ridge road for surveillance and monitoring. Despite this, only the former function has been assessed. This paper explores the function of the watchtowers along the Gask Ridge road using computational methods, including Monte Carlo hypothesis testing. The analytical approach, which is documented and reproducible with accompanying code, rejects that the watchtowers were randomly located along the Gask Ridge road, instead favouring the alternative hypothesis that the watchtowers were located to maximise the visibility of the road. Furthermore, it is possible to claim that the need to monitor the road shows a causal relationship with the location of the watchtowers, rather than associative. The findings support the interpretation that the main function of the watchtowers was for the surveillance and monitoring of the Gask Ridge road, providing an early warning system of an attack from the Highlands (Woolliscroft, 1993).