Earthworks located in the eastern part of Hamstead Marshall Park, West Berkshire, were the subject of a geophysical and topographic archaeological investigation. The survey area included the monument known as Castle I (two further castles are located in the parish) and parkland to the south. The methodology comprised an analytical survey of the archaeological earthwork remains, in addition to magnetometer and resistivity investigation. Geophysical survey identified several features visible as earthworks, in addition to possible pre-castle features. The earthwork survey revealed that the character of the large sub-circular earthwork is consistent with the existing interpretation of the monument as the remains of a siege castle. It may represent an unfinished work, as previously believed, but this survey raises a second possibility that it was built as a hybrid motte and bailey/ringwork form. The earthwork may have been associated with buildings or structures but it seems to have been occupied only briefly, supporting the idea that it originated as a siegework, perhaps dating to 1152–53, when King Stephen campaigned in the area. It is most likely that the castle was raised in order to besiege the medieval manorial centre located in the west of Hamstead Marshall Park (Castles II and III), the earthworks of which may represent the ‘lost’ castle of Newbury recorded by written sources. A post-medieval sham castle was later raised to the south-east of the site.
