This paper is the first attempt to assess the Isle of Axholme in the early medieval period, to determine its nature as a hitherto overlooked key point of economic, ritual, and geographic articulation along Northumbria’s southern frontier in the seventh century. First, an introduction to the current scholarly paradigm on the military campaigns of seventh-century English kings along this frontier is undertaken, followed by an analysis of recent interdisciplinary scholarship on the kingdom of Lindsey, and how this relates to Hatfield and the Isle of Axholme. The Isle is then used to discuss the locations of four major battles: the Battles of the River Idle, Heathfelth, Maserfield, and the River Trent. All but Maserfield can be reasonably argued to have taken place along the borders of the Isle, and in these cases, their locations are discussed. Still, an argument can be made to place Maserfield within this southern frontier too. As a result, this paper highlights the socio-economic and geopolitical importance of Lindsey, and reveals the Isle as an area of cyclical border conflict within a Humber-based frontier zone and, in doing so, offers a new perspective on Northumbria’s southern limits.