Complex sentence: The Fieschi Letter itself was written in the mid- 14th century, and some of the content is considered accurate by most historians, although other aspects of its story have been criticised as being simply unbelievable.
Split into simpler sentences: The Fieschi Letter itself is still believed to have been written in the mid- 14th century. Some of the content in the letter is considered accurate by most historians, but other aspects of the story have been criticised as being simply unbelievable.