Complex sentence: The walls may have been built as early as the 12th century, although it is more credible that they were constructed in 1438, and it may have had medical and administrative nature, as result of a plague epidemic that ravaged the city.
Split into simpler sentences: The walls may have been built as early as the 12th century, although it is more credible that they were constructed in 1438. It also may have had a medical and administrative nature due to a plague epidemic that ravaged the city.