Complex sentence: The complex retains the citadel-like walls setting it apart from the outside world, walls that contained a vast religious community--and today contain a more modest one--made up of the Convent of the Poor Clares and an order Grey Friars.
Split into simpler sentences: The complex retains the citadel-like walls setting it apart from the outside world. The walls contained a vast religious community, and today contain the more modest one Convent of the Poor Clares and a community of the Grey Friars.