The castle hill is an island in the reservoir of the Pļaviņas hydroelectric power plant, and it can be accessed by boat.  The ancient Selonian castle hill was settled several times between the 6th and the 12th century.  This was the political and military centre of the Selonian region.  In 1373, the Livonian Order built a stone castle on the hill, as it did on many other ancient hillocks.  The castle was sacked in 1704 during the Great Northern War.  Remnants of a square tower, a guard room and the 12 m embankment that once protected the castle are all that survive.  Approximately 300 m to the North of the Sēlpils castle hill is Oliņkalns hill, which is underwater.