The Roman castellum of Aardenburg lies strategically on a Pleistocene sandy ridge bordering the coastal plain in Zeeland, the Netherlands. The dynamic character of the coastal plain was rather attractive and offered the opportunity for specific economic activities. Although many archaeological excavations took place at Aardenburg, palaeoenvironmental research was limited to the macrobotanical analysis of a Roman well and a medieval pit. Two pollen boxes at two sites were recently sampled at Aardenburg, and 37 samples were studied for palynological and diatom analyses aiming at the reconstruction of the terrestrial and aquatic palaeoenvironments. At the base of the pollen box “Burchtstraat”, located on the edge of the defence ditch surrounding the Roman castellum, a peat bog corresponds to the Middle Holocene ombrotrophic peat layer of the coastal plain. This signal is not visible in the pollen box “Peurssensstraat” located more to the north. Instead, a “siliciclastic” peat formed dominated by trees and shrubs, and benthic brackish-marine diatoms (e.g., Diploneis didyma and Tryblionella navicularis), reflecting a mudflat. The overlying Roman waste layer records the same woodland-dominated landscape along with a disturbed environment. The diatoms are dominated by tychoplanktonic marine-brackish species (e.g., Paralia sulcata and Cymatosira belgica), indicating a marine influence. Furthermore, halophytes and dinoflagellate cysts are recorded in the Roman waste layer and increase in the above-lying marine clay deposits, suggesting the proximity of the coastal plain and an increase in tidal influence. The high percentages of spores (e.g., Sphagnum) are linked to peat excavation for fuel and to reworked peat eroded transported by tidal channel activity. The clay overlying the Middle Holocene peat bog at Burchtstraat holds marine and freshwater palynomorphs and diatoms, indicating that the ditch was receiving seawater via a tidal channel and freshwater most probably via the river Ee. This signal disappears in the overlying high medieval waste layer, where woodland and (reworked) peat dominate the pollen sum. The construction of ramparts in 1299 halted the marine influence. The coastal environment is also less visible in the presumably (post-) medieval top silt layer at Peurssensstraat, which holds high numbers of anthropogenic palynomorphs, suggesting that the area was already embanked.