Complex sentence: Already severely in decline by the 17th century, the cathedral was destroyed in the Great Fire of London of 1666, and the current domed St. Paul's Cathedral--in an English Baroque style--was subsequently erected on the site by Sir Christopher Wren.
Split into simpler sentences: Already severely in decline by the 17th century, the cathedral was destroyed in the Great Fire of London of 1666. The present domed St. Paul's Cathedral in the English Baroque style was erected on the site by Sir Christopher Wren.