Complex sentence: The library's original rooms, in which all books were chained to their cases in order to prevent theft, are no longer used as such, however they are still occasionally opened to host visiting lecturers' talks.
Split into simpler sentences: The library's original rooms, in which all books were chained to their cases in order to prevent theft, are no longer used as such. However, they are still occasionally opened to host visiting lecturers' talks.