Abstract: Mude, known today as Sint Anna ter Muiden (Zeeland, the Netherlands), was one of the outports of Bruges (northern Belgium) that developed along the Zwin tidal inlet during medieval times. The coastal plain of the medieval County of Flanders became embanked via the massive construction of dikes. Yet, little is known about the environmental conditions in which the medieval Zwin harbour region developed. Recent archaeological research allowed us to carry out the first environmental study of a complete sequence ranging from natural deposits to a raised soil associated with an embankment in the coastal plain using palynological and diatom analyses. The presence of tidal flat deposits influenced by a direct marine influx was confirmed. Sods from different origins, tidal flat deposits and a mixture of sediments and anthropogenic waste, were used to build an embankment. A layer holding a large number of shells was deposited on top of the sods likely for levelling and water management. The palynological assemblages from the sods and the shell-rich layer confirm human activity in an embanked area, such as the use of hemp for rope making and the digging of peat for fuel. Cultivated plants were grown in the area, while figs were imported from the Mediterranean region. The diatoms point to local brackish-freshwater conditions, indicating that the embankment was outside of the daily tidal reach by a tidal channel. A cultural soil developed on top of the embankment that was possibly used as a pathway for sheep herding, in a region famous for its wool production, while the signal of crops suggests the active use of the land in the vicinity.