The Fortress of Voorne, also known as the Jacoba Fortress, is in the city of Oostvoorne in the Dutch province of South-Holland. The name Jacoba Fortress comes from its use in the early 15th century by Jacqueline (Jacoba in Dutch!), Countess of Hainaut. She used it as her summer residence.
The oldest part of the fortress probably dates from the 12th century and it was probably used for strategic reasons. It was surrounded by a large moat and consisted of several buildings. Problems with the foundations led to the fortress becoming a ruin in the 17th century.
The ruins are now a monument.