Complex sentence: It was built by the Holy Roman Emperor Frederick II some time between 1240 and 1250, though it was probably never fully completed; in particular the interior fittings do not seem to have been finished.
Split into simpler sentences: It was built by the Holy Roman Emperor Frederick II some time between 1240 and 1250, though it was probably never fully completed. In particular the interior fittings do not seem to have been finished.