Complex sentence: Such areas were overcrowded, with low quality housing and little or no sanitation; poorly constructed dwellings, built with multiple stories, and which were often crammed into any area of open ground, created densely-populated areas of gloomy narrow streets and alleyways.
Split into simpler sentences: Such areas were overcrowded, with low-quality housing and little or no sanitation. Poorly constructed dwellings, built with multiple stories, and which were often crammed into any area of open ground, created densely-populated areas of gloomy narrow streets and alleyways.