The vallum in lacu (rampart by the lake) mentioned in 15th-century written sources as part of the Medieval landscape of Ostrowite (East Pomerania) has been researched by archaeologists and antiquaries since the 19th century. A wide range of noninvasive archaeological prospection methods were applied at Ostrowite in 2010-2015, including magnetic gradiometry, earth resistance, aerial photography, intensive field-walking, geochemical (phosphate) prospection, and the analysis of Airborne Laser Scanning. They were supplemented and verified by small-scale excavation work. This vast set of prospection methods was integrated into a Geographical Information System (GIS), and combined with an analysis of written sources, and allowed for the identification of a previously unknown ring-fort, which for the last 15 years has gone unnoticed by researchers conducting annual excavations in its vicinity. Its discovery and identification were only possible due to the integration of results from various methods, particularly non-invasive ones.