I've always been fascinated by linguistics - how it is that the same word can come up in different languages with the same meaning: English mother, German Mutter, Latin mater, Greek μήτηρ, Russian мать. But occasionally you get interesting variations of meaning for what is essentially the same word. The English word town is a cognate of Dutch tuin and German Zaun; but tuin means "garden" and Zaun means "fence". These must all come from a root word something like tūn, which is the Old English / Anglo-Saxon word for an enclosed space. So in English and Dutch, the word came to mean two different types of enclosure, one inhabited by people and the other by plants; whereas in German it came to mean the enclosure itself.
There aren't many cognates of this word outside the Germanic language family, but there is one: the Celtic root represented by the Irish word dún, meaning fort, and the Welsh word dinas meaning city, which crops up in all kinds of places. Old English / Anglo-Saxon adopted this word, in the context of hill-forts, to mean the hills themselves; and as tūn became our word "town", dūn became "down" as in the North and South Downs. (County Down is directly from the hill-fort at Downpatrick, Dún Pádraig.) The adverb describing a descent from the hill, "off the down", ofdūne, turned into adown and eventually became our adverb down. So "down" and "town" are originally the same word, a legacy of iron age enclosures and linguistic melding.