Complex sentence: By c.391 BC the city's overspill had overtaken the Aventine and the Campus Martius sufficiently to threaten its defences; around that year, a Gallic invasion overran and temporarily held the city.
Split into simpler sentences: By c.391 BC the city's overspill had overtaken the Aventine and the Campus Martius. This seems to have left the city vulnerable to attack; around that year, a Gallic invasion overran and temporarily held the city.