More than fifty years ago it was suggested that the stronghold of Lojsta slott in southern Gotland might be identical to a stronghold mentioned in a written source as Goltborch. The name has been translated as Gullborg and is probably one of the wooden castles erected by the troops of Queen Margaret of Denmark-Norway beginning in the winter of 1394/95. These were torn down in 1398, rebuiltin 1403, and finally conquered and destroyed by the Teutonic Order in 1404. As Lojsta slott has never been subject to archaeological excavation, the identification has remained an open question. This paper reports a recent underwaterarchaeological survey carried out in the waters around Lojsta slott, consisting of fieldwork that has included sampling for dendrochronological analysis. The dendrochronological results match the written sources very well and thus confirm the assumption that Lojsta slott is the stronghold of Gullborg mentioned in written sources. Moreover, the survey reveals that the underwater environment in the area has great archaeological potential.