And I dont want to spoil, but you asked if every season is about a different part town, no it isnt, basically every season has a theme of focusing on a different part of society (how he does it is very interesting in chancing focus, for example a character who had a very small role in earlier seasons, can suddenly be a main focus). With the drugs and the police still being quite part of it, underscoring or following it all the time.
But the themes for the seasons are (heard them before even watching, and wasnt a spoiler for me, you can be the judge for yourself). 1) Police/drugs (around in all seasons though), 2) The Docks, 3) Politics, 4) Schools, 5) Newspapers/Journalism.