Complex sentence: It is the site of the ancient city (polis) of Lappa, which became known in the Middle Ages as "Stimpolis" by reinterpretation as a single word ("in the city "), later simply"Polis".
Split into simpler sentences: It is the site of the ancient city (polis) of Lappa. In the Middle Ages, it was named "Stimpoli (s)" by reinterpretation as a single word ("in the city "), later simply"Polis".