Complex sentence: The fortress has been attacked many times, including by Simon de Montfort, without anyone actually succeeding in capturing it, and has also been used as a prison; the names of English Prisoners of War can still be seen on the old cell walls.
Split into simpler sentences: The fortress has been attacked many times without capture, including by Simon de Montfort. It has also been used as a prison; the names of English Prisoners of War can still be seen on the old cell walls.