Iron Age marsh-forts are large, monumental structures located in low-lying waterscapes. Although they share chronological and architectural similarities with their hillfort counterparts, their locations suggest that they may have played a specific and alternative role in Iron Age society. Despite the availability of a rich palaeoenvironmental archive at many sites, little is known about these enigmatic structures, and until recently, the only acknowledged candidate was the unusual, dual-enclosure monument at Sutton Common, near Doncaster. Assessing Iron Age Marsh-Forts considers marsh-forts as a separate phenomenon within Iron Age society through an understanding of their landscape context and palaeoenvironmental development. At the national level, a range of Iron Age wetland monuments has been compared to Sutton Common to generate a gazetteer of potential marsh-forts. At the local level, a multi-disciplinary case-study is presented of the Berth marsh-fort in North Shropshire, incorporating GIS-based landscape modelling and multi-proxy palaeoenvironmental analysis (plant macrofossils, beetles and pollen). The results of both the gazetteer and the Berth case-study challenge the view that marsh-forts are simply a topographical phenomenon. These substantial Iron Age monuments appear to have been deliberately constructed to control areas of marginal wetland and may have played an important role in the ritual landscape.