Complex sentence: The second part of the word is Celtic, "dūn (on)" meaning "lodgement, enclosure, or fortress", which is preserved, among other, as "-don" in the name of London.
Split into simpler sentences: The second part of the word is Celtic, "dūn (on)" meaning "lodgement, enclosure, or fortress". This is also preserved in other place names; for instance, as "-don" in the name of London.