During this, Jacob Kettler, the Duke of Courland, had in 1651 obtained from several native chiefs the cession of St Andrew's Island and land at Banyon Point (also known as Half-Die), Juffure and Gassan. Settlers, merchants and missionaries were sent out from Courland and forts were erected on St Andrew's Island and at Banyon Point. This was part of a period in Courlander history known as Couronian colonization, which also saw them colonise Tobago. The Courlanders believed that the possession of these territories would give them control over the river and enable them to levy tolls on all those who used the waterway. They erected a fort built out of local sandstone, appointed a Lutheran pastor, and positioned the cannons on the island so as to command both of the channels to the north and the south. The plan was to sell slaves to the colony in Tobago, but this did not prosper. In 1658, Kettler was made a prisoner by the Swedes during a war between Sweden and Poland. As a consequence, funds were no longer available to maintain the garrisons and settlements in the Gambia and in 1659, the Duke of Courland's agent at Amsterdam entered into an agreement with the Dutch West India Company whereby the Duke's possessions in the Gambia were handed over to the company.