Complex sentence: An important event in the history of Hull as a port was the acquisition by the crown (Edward I), followed, in 1297, by it becoming the only port from which goods could be exported overseas from the county of Yorkshire.
Split into simpler sentences: An important event in the history of Hull as a port was its acquisition by king (Edward I). In 1297, it then became the only port from which goods could be exported overseas from the county of Yorkshire.