Complex sentence: Some part of this structure can still be seen in the Cassaro District, that, indeed, was named after the walls themselves; the name Cassaro derives from the Arab al-qsr (castle, stronghold).
Split into simpler sentences: Some part of this structure can still be seen in the Cassaro district. This district was named after the walls themselves; the word Cassaro derives, indeed, from the Arab al-qsr (castle, stronghold).