Stone-built Roman forts commonly featured round towers placed at regular intervals along the outer walls. These towers helped stabilise the structure, as their thicker foundations supported the long defensive ramparts. Over time, tower shapes evolved into U‑forms that projected further outward to provide a better defensive angle, but in Maastricht the towers remained circular and were built nearly flush with the wall.
Five towers have been discovered to date, suggesting the castellum may originally have had as many as ten: one at each corner of the rectangular fort and the remainder spaced along its walls and in between the gates.