Complex sentence: The castle was founded by Bo Jonsson Grip and it is thought that the castle was completed sometime between 1373 and 1378 to protect Sweden's interests in southern Finland against the Hanseatic city of Reval (Tallinn).
Split into simpler sentences: The castle was founded by Bo Jonsson Grip and it is thought that the castle was completed sometime between 1373 and 1378. It's main purpose was to protect Sweden's interests in southern Finland against the Hanseatic city of Reval (Tallinn).