Complex sentence: Around 1560-80, most of the citizens, some 8.000 people, still lived on Stadsholmen, this central island at this time was densely settled and the city was now expanding on the surrounding ridges.
Split into simpler sentences: Around 1560-80, most of the citizens, some 8.000 people, still lived on Stadsholmen. This central island was at this time densely settled and the city was now expanding on the surrounding ridges.